Democrats and Republicans in the US Congress are as divided from each other as they have ever been. polarization has gripped almost every aspect of congressional life and increasingly interferes with members’ ability to fulfill their constitutional duties and shapes how they approach their jobs in almost every way. In the past few decades, scholars have shown partisan polarization’s dark cloud quells free-and-open debate in addressing pressing policy problems (Theriault 2008, 2013; Sinclair 2012; Curry 2015; Chaturvedi 2018; Howard and Owens 2019),Footnote 1 erodes Congress’s role on a variety of interbranch questions (Mann and Ornstein 2006; Whittington 2007; Lee 2008; Basinger and Mak 2010; Moore 2018; Lewallen 2020), and shapes how members communicate with their constituents (Cormack 2016; Russell 2018, 2020; Wang and Tucker 2020).

In this paper, we examine how partisanship shapes members travel on official trips abroad. Although the need for members of Congress to be cordial and civil with one another may seem like a trivial matter to many outside the institution, these norms existed for much of congressional history and most certainly the latter part of the 20th century during which bipartisanship rapidly expanded the federal government’s involvement in the lives of Americans (Jones et al. 2019). These policy achievements are at least in part thanks to the institutionalization of each chamber, which scholars have linked to norms of courtesy and reciprocity (Asher 1973; Matthews 1959; Polsby 1968). One needs only to read the final report from the landmark bipartisan Select Committee on the Modernization of Congress in the 116th Congress to get a sense for how the loss of civility has impacted members. One member of that committee, Representative Emanuel Cleaver (D-MO), notes in his opening remarks of the report:

Out of all of the recommendations made by this Committee, I’m proudest of our various recommendations that would serve the American people by promoting this level of bipartisanship and civility throughout the halls of Congress. If all of Congress could operate the way that the Modernization Committee has, the nation would be in a much better place (Kilmer and Graves 2020).

Moreover, the second chapter of the final report is entitled “Encourage Civility and Bipartisanship in Congress” and recommends, among other things, bipartisan retreats for members and their families at the opening of each Congress and more members-only spaces in the Capitol to encourage work across party lines (Kilmer and Graves 2020). A persistent fear that members have in the modern Congress is being seen with members of the other party fearing that such optics suggests ideological compromise that may make them vulnerable to primary election challengers (McGee and Moniz 2021). The backlash against Senator Dianne Feinstein (D-CA) for hugging Judiciary Committee chairman Lindsey Graham (R-SC) following the Supreme Court confirmation hearings for Amy Coney Barrett provides a salient example of this threat.

By analyzing Congressional Delegation (CODELs), that is, official congressional travel abroad, from 1977 to 2018, we ascertain which members are willing to travel abroad with members of the other party and which members will not. An analysis of these trips indicates the members willing to make themselves available to help weave a bipartisan social fabric to facilitate the necessary goodwill and trust to legislate, and the members who will not be bothered. While we cannot say for certain that all members who go abroad with members from the other party care about the social fabric of Congress and that all members who won’t don’t, their CODEL activity at least informs us who makes themselves available to it and those who won’t.

The paper proceeds as follows. First, we explore the literature on polarization and congressional travel. Second, we introduce the data set and discuss our research design. Third, we test our hypotheses. While we find that data experiences ups and downs across time, the trend in partisan country-days is increasing. We take this to mean we are still in the early days of this trend. We find evidence that liberal Democrat spend more country-days abroad with copartisans than do moderate Democrat. Moreover, we find that ideologically extreme members from both parties spend fewer bipartisan country-days abroad than their ideologically moderate colleagues. Taken together, these results suggest that ideological extremism may feed the partisan warfare that is continuing to fray the social fabric of Congress in a variety of ways within each party and across them as well. Furthermore, it indicates that an additional cost of party polarization is the decreased opportunity of members from across the aisle to get to know each other while they are traveling abroad on official business. Finally, we explore the implication of our findings.

Partisan polarization and congressional travel

Congressional travel abroad, in an official capacity, was unknown until 1936 when Representative Thomas S. McMillan (D-SC), then the chair of the appropriations subcommittee responsible for funding the state department, visited almost 30 diplomatic and consular missions in an inspection of the US Foreign Service (Carnahan 1953). From that point on, travel among members of Congress exhibited a consistent upward trend. In the earliest years, travel abroad was concentrated mostly among members of the foreign affairs committee and its subcommittees (Carnahan 1953). Today, most members of Congress go abroad at some point in their career whether it is on an official trip, it is sponsored by a committee, or it is a privately sponsored one (e.g., Rosenson 2009; McGee and Moniz 2021).

Members of Congress travel for a number of different reasons. The earliest works suggest members gained valuable on-the-ground experience observing how foreign aid was being spent, providing a check on information furnished by executive-branch officials, and promoting good will with US allies (Carnahan 1953). It has even been argued that “the overwhelming endorsement of the Marshall Plan by the Congress is traceable, in large measure, to the legislative expert guidance” from a select committee on foreign aid that conducted extensive European travels (Carnahan 1953, p. 123). More recently, scholars have argued that members travel for educational purposes as well, attending conferences and policy roundtables (Rosenson 2009; McGee and Moniz 2021) or even for party-building purposes (Rosenson 2009).

Conceptualizing travel as a medium for member socialization is a relatively new theoretical development (Alduncin et al. 2014, 2017; McGee and Moniz 2021; Curry and Roberts 2020) that has added to a rich literature about the social connections between members. Scholars have linked members based on supporting legislative initiatives by cosponsoring legislation (Fowler 2006; Kirkland and Gross 2014), co-signing “Dear Colleague” letters (Box-Steffensmeier et al. 2019; Craig 2020), transferring campaign funds between members (McGee 2017; Box‐Steffensmeier et al. 2020), participating in joint press events (Desmarais et al. 2015), wearing seer-sucker suits or playing baseball and softball (Lawless et al. 2018), and even joining internal groups like congressional caucuses (Victor and Ringe 2009; Ringe et al. 2013). These non-travel focused studies do not tend to theoretically associate members’ connections to one another with an increase in civility or collegiality between them.Footnote 2 The social situation that foreign travel produces among those traveling, on the other hand, is unique and central to the conceptualization of our data. Whereas members may not even need to speak to one another to get their names on the same bill or letter, instead relying on their staffers to fulfill these tasks, congressional travel can involve days together on a trip away from the toxic Washington, D.C., political environment. In some instances, travel can be to regions wherein the members and those facilitating the trip may be the only people around who speak English (McGee and Moniz 2021). This produces a circumstance where members either sit in silence for days on end or they strike up a conversation with the member sitting next to them.

To appreciate the toll that partisan polarization has taken on the social fabric of Congress, it would behoove of us to first recount the status quo within Congress during the middle of the 20th century when polarization was at a notable low. Nelson Polsby (1968, p. 167) quotes Representative Clem Miller (D-CA) in describing the House of Representatives:

One’s overwhelming first impression as a member of Congress is the aura of friendliness that surrounds the life of a congressman. No wonder that ‘few die and none resign.’ Almost everyone is unfailingly polite and courteous. Window washers, clerks, senators-it cuts all ways. We live in a cocoon of good feeling.

Similarly, Donald Matthews (1959) quotes one senator describing the Senate as akin to a small town with great pressures for conformity; Matthews goes on to illuminate a variety of expectations for cordiality and respect among members as being cornerstones of the folkways of Senate life.

While “the club” atmosphere has been more pervasive in the Senate, the House was an integral part of the Washington community that flourished in mid-twentieth century. As members traded in their weekend dinner parties with other members to rush back to their constituencies, the ties that bound members together—even those across the aisle—began to break: their wives were no longer having tea together while their men legislated and their boys were no longer playing on the same little league team. Nonetheless, CODELs persisted.

A report from the Association of Former Members of Congress highlights the importance of these personal relationships: “If there’s a ‘secret sauce’ to building consensus, solving problems, passing legislation, and getting anything done, it’s ‘the personal relationships that you develop.’ Getting to know other Members may be the most effective antidote to the partisanship” (Sobol and Steinhorn 2020, p. 10). The report quoted another: “You work on relationships about 100% of the time, about 10% of the time you get to work on legislation” (10). Another responded: “You don’t do deals with people you don’t trust. You don’t trust people you don’t know” (10). The report’s first recommendation was for “leadership [to] encourage more participation on CODELs” (33). As the quote from one Democratic member illustrates: “You really do get to know people on these trips… [You] get to meet parliamentarians and businesspeople and see things. But, the most important thing to happen at every CODEL is I get to be friendly with one or more of the attending Republicans” (33). Of course, that only works for members who are willing to travel with members from the other side of the aisle.

CODELs are so important because so many of the other bridges that used to connect the parties have collapsed, and the atmosphere on Capitol Hill is not one of rebuilding those bridges. Partisan polarization has certainly contributed to social disintegration in Congress. Reforms in the mid-20th century shifted power to party leaders (Rohde 1991; Sinclair 2006), which in turn enabled individual members to prioritize their political interests at the expense of institutional ones (Theriault and Rohde 2011; Theriault 2013). As the committee system waned in power and the selection of its leaders became less about seniority and more about the ability to raise money (Green and Harris 2007; Cann 2008; Currinder 2008), party leaders only further enhanced their power and by creating a less transparent legislative process that excluded almost all but the rank-and-file and select committee chairs from the process of writing bills (Sinclair 2012; Curry 2015). With an increasing number of members left out of the core duties of serving in Congress, institutional patriotism collapsed. This collapse only intensified the trend of members running against the institution of Congress and ceding more and more power to the executive and judicial branches (Fenno 1978; Mann and Ornstein 2006; Whittington 2007; Moore 2018). This process fed back on itself too with elections at all levels becoming nationalized and the pressure to retain or gain majority party status raising the stakes for party leaders to deliver results or step aside (Lee 2009, 2016; Abramowitz and Webster 2016; Koger and Lebo 2017; Lewallen and Theriault 2012; Carson et al. 2020). Taken together, this hostility among party leaders on the national stage permeated throughout the legislative parties and was then reinforced by activists threatening incumbents in primaries who were seen of as being too cordial to the other political party.

Members, too, affirm the plague of partisan polarization on collegiality in Congress. In her exit interview with the Library of Congress, Senator Barbara Mikulski (D-MD), in response to a question about how Congress and relationships among its members have changed over her four-decade career spanning both the House and Senate, she stated firmly:Because the study of CODELs

I feel that partisanship has become more hyper and more prickly. I don’t think that’s a good thing. You know, we’re here to represent states. We’re not here to represent political parties. We declare our party, because each party has a platform, and a philosophy, but at the end of the day, people want us to be able to come together for the common good and the good of the country. We’ve tried to do that among the women (Mikulski 2016).

Senator Mikulski has notably, throughout her career, hosted the female senators of both parties regularly behind closed doors in part to facilitate relationships among the women without the political pressure generated from the Capitol Hill press. Today, these have traditions not only waned, but members pride themselves on not even renting an apartment in Washington. Representative Paul Ryan (R-WI), even after becoming speaker, slept on a cot in his office. Clearly members themselves and academics alike see value in building meaningful social and professional relationships with their colleagues (Lawless et al. 2018). Civility may coexist with polarization (Theriault 2015), and CODELs may very well be one of the more sustained avenues to build these relationships. We evaluate the extent to which this avenue continues to exist and for whom.

Partisan travel data

Our data set contains all publicly funded foreign trips taken by House members from the 95th through the 115th Congresses (1977–2018).Footnote 3 Travel data include the country visited, dates of the trip, the members of Congress involved, and the committees or delegations sponsoring the travel. We then reorganized these data into observations based on days spent in a country, or “country-days,” as they are commonly termed because “country-trips” is too rigid a measure to pick up the members who only go on part of a trip (see also Alduncin et al. 2014, 2017). For example, a congressional delegation to China from June 21 to 24, 2010, is broken down into four country-days: June 21-China; June 22-China; June 23-China; June 24-China. Following previous scholars, we then categorize each country-day into three groups: solitary travel (when one member travels alone), partisan travel (when all members traveling together are of the same party), and bipartisan travel (when the CODEL features members from both parties).

Total country-days across congresses varies from just under 3,000 to slightly more than 8,000, though members usually spend between 4,000 and 5,000 country-days abroad (see Fig. 1). The overall number of days abroad overwhelms two interesting trends in partisan country-days. First, from the 95th to the 109th Congress, the trend stays relatively flat (see Fig. 2). Beginning in the 110th Congress, though, the trend starts growing rather rapidly. This increasing trend includes congresses with Democratic and Republican majorities suggesting that it is impervious to which party enjoyed a majority. Overall, while the data itself is not a consistent upward increase every congress the larger trend is, and we expect it to resume its upward trajectory once the COVID-19 pandemic subsides.

Fig. 1
figure 1

Aggregate country-days by Congress, 95th–115th Congresses

Fig. 2
figure 2

Percent of country-days spent on partisan travel by Congress, 95th–115th Congresses

This increase in travel abroad in single-party travel is just another example of the increasingly polarizing Congress. While polarization does not guarantee incivility, it certainly does make it easier. And, that fraying of the social fabric in Congress is aided by the replacement of members who were familiar with the pre-polarization Congress with those who enter Congress and are subject to partisan warfare from day one. The era of the bipartisan orientation for freshmen members is over and has been replaced not only with party leader led orientations and retreats but is also supplemented by outside groups and think tanks that also sponsor partisan orientations, talks, and events for new and senior members alike (Mann and Ornstein 2006; McGee and Moniz 2021). If members start their congressional careers believing that trips composed only of copartisans is not only acceptable but increasingly the norm, we should expect this trend to accelerate in the coming years due to the same logic used to explain partisan polarization more generally (Theriault 2008).

Hypotheses

Because the study of CODELs is not entirely new, we do not retread the ground covered by the previous studies (Alduncin et al. 2014, 2017; McGee and Moniz 2021; Curry and Roberts 2020). In this paper, we add to this literature by examining the specific role that ideology and intraparty caucus Rep. Study Committee have in who goes abroad and with whom. To provide a comprehensive landscape for CODELs, we examine three different dependent variables. First, we analyze each members’ total number of country-days abroad. Second, we examine the number of bipartisan country-days abroad. Finally, we examine the number of partisan country-days abroad. Because the first of these dependent variables has received the most attention, we offer our analysis on them only to show the consistency across studies and as a benchmark for bipartisan and partisan country-days, which is where we focus our attention.

As with most other activities in Congress, we believe ideology motivates travel abroad. We expect more ideologically extreme members to spend more time abroad than their moderate colleagues with only their copartisans. By doing so, they signal an ideological purity to their most fervent political supporters, both in Washington, D.C., and in their constituencies.

Ideologically Extreme Hypothesis: More ideologically extreme members spend more days abroad with only copartisans.

Similarly, we should expect more moderate members to spend more time abroad with members of the opposite party signaling to their constituents that they can still make connections across the aisle beyond what is revealed in roll-call votes.

Ideologically Moderate Hypothesis: More ideologically moderate members spend more days abroad with members of the opposite party.

While these hypotheses may seem like flip sides of the same coin, they examine two different dependent variables. The Ideologically Extreme Hypothesis is tested on the number of copartisan country-days abroad whereas the Ideologically Moderate Hypothesis is tested on the number of crosspartisan country-days abroad. It is possible that a frequent CODEL traveler could spend lots of days abroad with both their fellow partisans and those from the other side of the aisle. Including both hypotheses allows us to test for this situation.

In the past few decades, not only we have seen dinner groups like Senator Mikulski’s decline, but we have also seen more and more members flocking to ideological intraparty factions such as the Congressional Progressive Caucus or the Republican Study Committee (Thomsen 2017; McGee 2020). These groups represent different ideological visions for policymaking within the House of Representatives and currently all ideological perspectives from liberal Democrat to conservative Republicans have at least one group to call home and some ideologies have multiple options. Rep. Study Committee in these factions has been shown to impact the composition of a member’s campaign donors (Clarke 2020), the distribution of their campaign funds to their fellow members (Bloch Rubin 2017; McGee 2017), and even the success or failure of their legislative priorities (Green 2019; McGee 2020). Importantly for this paper, Rep. Study Committee in these groups can serve as a credential above and beyond ideology to both outside and inside players in the political process (Theriault 2008; Lynch and Madonna 2013; Lee 2018). Furthermore, these groups have been known to have regular meetings behind closed doors and even to screen potential members before being admitted entry (McGee 2020). Some groups even require commitments such as funds from members’ campaign or office accounts or even commitments to being bound to a certain number of floor votes by the group in each Congress (Green 2019; McGee 2020).

Intraparty factions could also alter who travels with whom and, hence, the social dynamics in Congress. More likely than not we should expect these groups to reinforce the actions just hypothesized where members who gravitate toward more moderate (extreme) factions are more likely to join in on bipartisan (partisan) trips. Beyond providing a separate mechanism for operationalizing partisan polarization, factions also provide a different window into member behavior altogether. A growing body of work finds that the House Freedom Caucus, the most ideologically extreme faction in the House Republican Party exhibits a unique approach to governance that makes it stand out especially in this era of partisan warfare. When it comes to obstructing governance, refusing to compromise and deploying bold displays or ideological purity, no group is more distinct than the House Freedom Caucus. In fact, it was formed as an explicit criticism of the other dominant conservative faction at the time, the Republican Study Committee (Wallner 2017). Between its founding in the 114th Congress and today its members contributed to ousting a Speaker of the House (Green and Bee 2017), exerting significant negative agenda control (Green 2019; McGee 2020), and rising to the highest ranks of President Trump’s inner circle. While members of the House Freedom Caucus do create headaches for their own party in an attempt to keep it pure, the old adage of your enemy’s enemy being your friend is not followed. Indeed, the purest of the House Freedom Caucus, if they condone congressional travel at all, are far more likely to travel with the Republican establishment than they are Democrat. Taken together with the research on asymmetric polarization that suggests Republicans have shifted to the right more than Democrat have to the left, in part due to pressure from party activists (Grossmann and Hopkins 2016; Theriault 2008, 2013), we should expect the largest effects on travel with only copartisans to come from members of the House Freedom Caucus.

Warrior Hypothesis: Members of the House Freedom Caucus will spend more days abroad with only copartisans.

Because of the distinctiveness of the House Freedom Caucus, we think it is the intraparty caucus most likely to affect member travel behavior. We include the other intraparty caucuses for completeness sake, but we think of the Freedom Caucus as an easy case. Including it in the same model as ideology, though, subjects it to a difficult test. If more conservative Republicans are more likely to join the House Freedom Caucus and if our ideological hypotheses are accurate, then a model that includes both tests to see if being in the House Freedom Caucus influences your travel abroad above and beyond your ideology making it a difficult test. In combination, this Hypothesis is the easy case for a difficult test.

Multivariate analysis

We present the summary statistics for the key variables in our analysis in Table 1. Because each of the dependent variables is over-dispersed, we choose to employ multivariate negative binomial regression analysis. The intraparty caucus data are based upon only when the caucuses were in existence, which in every case is less than the entire span of the data for the other variables.

Table 1 Summary statistics of relevant variables

Our three dependent variables also have an exceptional number of zeros (see Fig. 3). The mere presence of exceptional zeros does not alone mean that a different model must be used, though the zeros in our data are generated in distinct ways. In some cases, members may choose not to travel explicitly for personal or ideological reasons. These members would therefore produce a set of “certain zeros” because they are never going to accept an invitation to travel. Other members on the other hand may have differing probabilities of traveling based on their committee appointments or personal network of relationships in their chamber. If these members do not travel, it would be for a wholly different set of theoretical reasons and therefore these zeros emerge from a separate data generating process (King 1998). Given the distribution of the dependent variables, the data generating processes just outlined, and following other scholars in this area (e.g., Alduncin et al. 2017), we estimate a zero-inflated negative binomial model.

Fig. 3
figure 3

Histogram of total country-days abroad

This modeling strategy leads to two sets of estimates per model. First, zero-inflated negative binomial models estimate a logit equation which produces the likelihood of a member falling into the certain zero category (i.e., those members with personal or ideological reasons for never traveling or those members because of their seniority status or committee assignment are never asked to travel abroad). These estimates are interpreted substantively such that a positive coefficient suggests an increased probability of a certain zero. The second set of estimates come from the negative binomial and are conditional on a member not being a certain zero. Therefore, the negative binomial estimates that are positive are associated with more country-days of travel. While one can choose subsets of variables for the logit and negative binomial equations, we argue that the decisions about whether to travel and for how long one travels are impacted by the same set of institutional, personal, and political factors. Therefore, we use the same set of independent variables for both the logit and negative binomial equations.

Our chief independent variable is Ideological Extremity, which is calculated as the absolute value of the member’s first dimension DW-NOMINATE score. To control for institutional factors, we also include variables indicating whether the member sits on the “power” committees of appropriations, rules, or ways and means, as well as servicing on the armed services or foreign affairs committees. We expect more powerful members (i.e., those on the “big three”) and members whose committees deal with foreign policy concerns to be more likely to travel. Committees dealing with public policy surrounding foreign aid and military activity, especially foreign affairs, have historically been frequent funders of travel (Carnahan 1953). And, as leadership in the House continues to gain power over the past couple of decades (Curry 2015; Theriault 2008), we would expect members of influential committees—who themselves are on the outskirts of leadership if not formally members—to also have disproportionate opportunities to travel.

With an eye toward members’ personal ambitions, chiefly their proximate goal of reelection (Mayhew 1974), we include the member’s vote-share in the election entering the congress. Safer members should feel more comfortable traveling more often. If a member has significant support from their constituents, it is easier to convince them that fact-finding missions abroad are necessary for improving their job performance (McGee and Moniz 2021), or, if not, the punishment that constituents might inflict upon members who excessively travel would be less costly to electorally safe members. We also include the number of terms a member has served in the House of Representatives. More senior members have had more opportunities to travel and also be more likely to have accrued more area-specific policy expertise. Members who choose to pursue good public policy as a goal seek out opportunities to travel to assess the effect of their policymaking abroad. Other scholars have shown evidence that certain types of travel can actually lead to members being more effective lawmakers and that trips can include policy specific information valuable for members who seek additional context or expertise (Moniz and McGee 2021).

Finally, we include a set of demographic variables to attempt to understand how descriptive features of members might impact their travel behavior. Therefore we include binary variables indicating if a member is Black, Latino, or female. It is possible that members might be less inclined to travel if they are nonwhite or female because of concerns about how they might be perceived by their constituents when they travel. We also choose to run separate analyses on Democrat and Republicans as well as include clustered standard errors by member in all models.

While this base model is sufficient for testing both the Ideologically Extreme Hypothesis and the Ideologically Moderate Hypothesis, we must include additional variables to test the Warrior Hypothesis. These models are the same as the base model except they also include binary variables indicating whether a member is part of each faction within their respective party. Members can belong to more than one faction. The Democratic factions include the Congressional Progressive Caucus (liberal), the New Democrat Coalition (moderate), and the Blue Dog Coalition (moderate). The Republican factions include the House Freedom Caucus (conservative), the Tea Party Caucus (conservative), the Republican Study Committee (conservative), and the Main Street Coalition (moderate). Due to limits in faction Rep. Study Committee rosters, these models are restricted only to data from the 107th through the 115th Congresses. Note that, for the Republican factions, multiple models are run because the Tea Party Caucus forms in the 111th Congress and the House Freedom Caucus forms in the 114th Congress, so those models are based on even less data.

Before modeling partisan or bipartisan country-days abroad, it is helpful to see what patterns emerge when we analyze all trips together (see Table 2). By conducting this initial analysis, we are more equipped to understand which factors drive travel in general as well as how those factors impact decisions about taking trips with members from their own, or the other, party. From the start, we see negative and significant logit coefficients for many of the factors that we might expect to drive travel decisions, and in almost every case (across both the Democratic and Republican models), those same variables have significant and positive–negative binomial coefficients. Taken together such a pairing of coefficients indicates both that these variables are associated with members being less likely to reject travel altogether (i.e., not be “certain zeros”) and take more country-days abroad.

Table 2 Examining the likelihood of participating in CODELs, 95th–115th congresses

The factors we identify here are common culprits in the congressional travel literature; safer members, more senior members, and members associated with powerful committees or those committees with jurisdiction over foreign or defense policy all are associated with increased travel. While none of these findings are surprising, they do produce a useful benchmark as we start to analyze the partisan and bipartisan trips. Besides those variables just identified, we also see Latino Democrat taking more trips abroad and female Democrat taking fewer trips abroad. While the former finding is a bit of a surprise, the finding for female members is in line with our expectations because, perhaps, women fear receiving more scrutiny for engaging in corrupt or questionable behavior. Furthermore, it behooves us to note that ideological extremity demonstrates no significant relationship with travel in general. This finding situates our argument that ideological extremism is a relevant factor mainly when considering with whom members are traveling, rather than as a factor leading them to want to travel at all.

Having established a useful set of baseline findings, we next turn to patterns in partisan member trips abroad (see Table 3). Among both Democrat and Republicans, we see more ideologically extreme members spending more country-days abroad with only copartisans. This finding provides support for our Ideologically Extreme Hypothesis. For Democrat, the most ideologically extreme members travel about five country-days with their copartisans (see Fig. 4). The Democrat with an average ideology score spends just two country-days abroad with copartisans. For Republicans, the effects are very similar with the most extreme member traveling with copartisans for about four country-days, and the average remaining just under two country-days (see Fig. 5). Taken together, these findings suggest that the most extreme members within each party are traveling with only copartisans twice as much as their most moderate colleagues.

Table 3 Examining the likelihood of participating in partisan CODELs, 95th–115th congresses
Fig. 4
figure 4

Impact of ideological extremity on partisan country-days abroad for democrat

Fig. 5
figure 5

Impact of ideological extremity on partisan country-days abroad for republicans

We also find a familiar set of negative logit coefficients, indicating a decreased likelihood of rejecting travel altogether, associated with safer members, members on the armed services committee, and members on the foreign affairs committee across both models. Only Democrat on the foreign affairs committee also increased their partisan travel abroad though. It is unclear why there is a differential effect here among Democrat and Republicans. Foreign policy tends to be one of the more conciliatory areas of public policy when it comes to partisanship. Yet, perhaps we are seeing that even that era is starting to fade. More senior Republicans and Black Republicans also travel more with their fellow partisans (the latter contains so few members that caution should be used in interpreting it). It is puzzling to see more senior Republicans embracing more partisan travel since much of the discourse around the contemporary Republican Party suggests that its newer members are to blame for the more raucous behavior exhibited by their caucus (Theriault 2008, 2013; Green 2019).

Next, we examine bipartisan travel behavior. We, again, find safer, more senior, and members from the powerful or travel-inclined committees driving the results. Given that we did not see these variables driving our partisan-only results (except for more senior Republicans and Democrat on the foreign affairs committee), it is not a surprise to see them stand out here. This set of results is therefore consistent with the total travel results from Table 1. We also find support for the Ideologically Moderate Hypothesis, wherein more moderate members are more likely to spend more country-days abroad with members of the opposite party, among Democrat but not Republicans. The most moderate Democrat spend almost 11 country-days abroad with Republicans compared to only about five among their most extreme counterparts (see Fig. 6). This finding indicates that moderate Democrat are traveling about twice as much with Republicans compared to their most extreme colleagues, which is again suggestive of congressional social networks being infected by ideologically charged discontent. This finding is also consistent with the asymmetric polarization of the American party system wherein Republicans having been driven farther to the right by the contemporary conservative ideological movement (Grossmann and Hopkins 2016; Hacker and Pierson 2006; Theriault 2008). Since Democrat are more likely to garner support from diverse interests, it makes more sense to see their more moderate members clinging to bipartisanship and the vestiges of compromise. Republican Latino members appear less likely to reject travel altogether, yet this was also true in the partisan travel model. Democratic Latino members do take more bipartisan trips abroad and this type of travel seems to be driving the finding from the total travel models in Table 1.

Fig. 6
figure 6

Impact of ideological extremity on bipartisan country-days abroad for Democrat

Overall, we can also conclude that, with occasional exceptions for Democrat on the foreign affairs committee and more senior Republicans, the traditional factors associated with increased travel among members of Congress seem to be driving bipartisan travel most. Despite this finding, we have also identified a new relationship among ideologues in each party and increased trips abroad with only their copartisans. Yet, we have also proposed hypotheses related to travel that may be shaped in part by ideology but also by the social connections members form with one another. As these intraparty factions are closeddoor and ideologically focused, we expect membership to reinforce the patterns in travel behavior that ideological extremism produces in general. That is, moderate groups like the New Democrat, Blue Dogs, and Main Street Coalition should encourage bipartisan country-days abroad because these groups often note their interest in forming governing coalitions across the aisle (McGee 2020). On the other hand, the ideological extremism of members in the Progressive Caucus, Republican Study Committee, Tea Party Caucus, and Freedom Caucus are likely to produce increases in partisan country-days abroad. In particular, we expect the largest effects to come from the Freedom Caucus, because of its distinct reputation as a home base for true warriors in the House Republican Party (Table 4).

Table 4 Examining the likelihood of participating in bipartisan CODELs, 95th–115th congresses

Among Democrat, we find that service on the foreign affairs committee is still oddly driving increased partisan travel (see Table 5). We also see members of the Blue Dog Coalition being less likely to reject partisan travel outright, which is at odds with our expectations. Yet, we do not see the negative binomial coefficient with a significant effect so we cannot also say we are seeing them go on increased travel. In fact, looking at the bipartisan travel model, we see relatively large effects for members in the more moderate factions (i.e., the Blue Dogs and New Democrat) traveling more country-day abroad with members of the opposite party. Members of the Blue Dog Coalition spend about 16 country-days abroad with Republicans compared to only about eight among nonmembers (see Fig. 7).

Table 5 Examining the likelihood of participating in CODELs with democratic faction Rep. Study Committee, 107th–115th congresses
Fig. 7
figure 7

Impact of Rep. Study Committee in the Blue Dog Coalition on bipartisan country-days abroad for Democrats

The effect size among the New Democrat is smaller, but still significant. Members of the New Democrat spend about 12 country-days abroad with Republicans compared to nonmembers who only spend about nine (see Fig. 8). These findings are consistent with our expectations that more moderate members would prefer bipartisan as many of them focus on building coalitions across the aisle in favor of governance. It also makes sense to see the larger effect sizes coming from the Blue Dog Coalition, which is a smaller and more tight-knit faction compared to the New Democrat Coalition. Other factors we have seen before among Democrat pursuing bipartisan travel still appear to be playing a role. That is, we still see safer members, more senior members, Latino Democrat, and members of the foreign affairs committee taking more country-days abroad with Republicans.

Fig. 8
figure 8

Impact of Rep. Study Committee in the New Democrat Coalition on bipartisan country-days abroad for Democrats

Shifting now to the Republicans, we discuss the results as broadly as possible given the unorthodox number of models required due to the different founding dates of the various conservative factions throughout the period of analysis. Looking first at partisan travel (see Table 6), we find that members of the Republican Main Street Coalition are less likely to take trips abroad with their copartisans, at least in one of the models. nature of the finding though it is easily quibbled with. We see some evidence again of more senior Republicans being more likely to travel with copartisans. Generally speaking, the most consistent results relate to those factors related to whether or not a member rejects travel outright, which is not the main focus of this paper nor are the results significantly different from the previous models discussed.

Table 6 Examining the likelihood of participating in partisan CODELs with republican faction Rep. Study Committee, 107th–115th congresses. The tea party model only examines data from the 111th-115th congresses and the House Freedom Caucus model only examines data from the 114th and 115th congresses

Turning to bipartisan travel (see Table 7), we do present some consistent results in the direction we expect for the conservative factions. As soon as it is founded, we see members of Tea Party Caucus are less likely to take bipartisan trips abroad. In the Tea Party Model, we see members of the Tea Party Caucus spending about seven country-days abroad with Democrat compared to about nine country-days for nonmembers (see Fig. 9). The effect size is smaller in the Freedom Caucus model, with Tea Party members spending just under 10 country-days abroad with Democrat as compared with about 11 for nonmembers (see Fig. 10).

Table 7 Examining the likelihood of participating in bipartisan CODELs with republican faction Rep. Study Committee, 107th–115th congresses. The tea party model only examines data from the 111th-115th congresses and the freedom caucus model only examines data from the 114th and 115th congresses
Fig. 9
figure 9

Impact of Rep. Study Committee in the Tea Party Caucus on bipartisan country-days abroad for Republicans, tea party model

Fig. 10
figure 10

Impact of Rep. Study Committee in the Tea Party Caucus on bipartisan country-days abroad for Republicans, freedom caucus model

We also find members of the House Freedom Caucus being more likely to outright reject any bipartisan travel. While we do not see the expected increase in partisan travel from these members, the evidence that they are less likely to take bipartisan trips—or even consider them in the case of the HFC—is a step toward evidence of our Warriors Hypothesis. Oddly, we also find a single instance of the RSC being more likely to engage in bipartisan travel, but like the Main Street Coalition finding from the previous set of models it fades and therefore is on flimsy ground to be asserted as a solid finding. The drivers of bipartisan travel we have seen before again show up with safer, more senior, and certain committee service improving the likelihood of taking trips with those across the aisle.

Taken together, the results from highlighting factions largely reinforced our findings from the initial modeling. Moderate Democratic intraparty caucus members spend more time abroad on bipartisan trips and conservative Republican intraparty caucus members tend to stay away from bipartisan trips. While we had hoped to find that factions were also reinforcing the disposition of ideological extremists to travel among their own, we did still uncover that relationship when utilizing our scores derived from DW-NOMINATE. Overall, our results also bolster already existing findings about the institutional, demographic, and district factors that drive the desire of members of Congress to travel. The results suggest that we were accurate in describing the House Freedom Caucus as the easiest case in a difficult test. The substantive results were generally pretty weak for the intraparty caucus Rep. Study Committee, but were strongest for the House Freedom Caucus.

Discussion

What members have held out as the last hope for developing cross-party relationships seems to increasingly be becoming another victim of party polarization in Congress. Although the bipartisan nature of traveling abroad persisted longer than many of the other threads of the congressional social fabric, our analysis shows that it, too, is fraying. We show that from the 95th to 115th Congresses members travel increasingly with only members of their own party. While the trend line is clear, the consistent data points of increasing partisan trips are only present starting with the 110th Congress. Taken together we are likely still on the earlier end of this growing trend among members. The continued replacement of members who were familiar with the pre-polarization Congress and its socialization norms with those who enter Congress and are immediately indoctrinated into a partisan warfare mentality is serving to aid this trend. Moreover, the decline of bipartisan orientations, retreats, and groups within Congress makes it increasingly easier for members to start their congressional careers believing that trips abroad composed only of copartisans is not only acceptable but the norm.

Using multivariate regression analysis, we present evidence of ideological extremity among members contributing to increased country-days abroad with only copartisans. These findings were evident among both Republicans and Democrat, and in both cases, it was one of the only explanatory factors explaining travel with only copartisans. The effect size was significant as well, with ideological extremity effectively doubling the number of country-days spent abroad with only copartisans. When examining bipartisan country-days abroad, we found complementary evidence for Democrat that more extreme members were less likely to spend extended periods abroad with members of the opposite party. Underpinning these results was a set of consistent findings that safer members, more senior members, and members who serve on powerful or foreign policy committees all travel more, and usually they travel with members of the opposite party.

In an effort to conceptualize ideology in an alternative way that also gets at the social fabric of Congress, we used Rep. Study Committee in ideological intraparty factions among members of Congress to explain trip-taking behavior too. Democrat who are members of the moderate factions (i.e., the Blue Dog Coalition and the New Democrat Coalition) are more likely to take bipartisan trips. Republican members who join the ideologically conservative factions on the other hand tend to stay away from bipartisan trips abroad. While our faction models did not produce explicit evidence in favor of increased partisan travel among their party’s ideologues, the avoidance of bipartisan trips—especially among Republicans—suggests we are on the right track. Keeping in mind that we expect these trends to be young still today, more time may provide the additional evidence to substantiate our argument.

These results paint a complex picture that requires not only additional analysis of these data, but also a supplement to these data. Only in the final six congresses do we see consistent growth in partisan country-day abroad. If this trend continues, the behavior of members and the impact of the process of member replacement described here will result in a clearer picture. For now, what is clear is that even if ideologically extreme members are not explicitly increasing their number of partisan country-days abroad, their ideological leanings are keeping them from engaging in as many bipartisan country-days abroad as they have in the past. Many different factors are likely contributing to this, but polarization, driven internally in Congress by party leaders and externally by activists, voters, and interest groups, is at least partially responsible.