This result at first seems at odds with the partisan rancor that has engulfed Washington, and GOP lawmakers’ embrace of President Trump’s unorthodox policies and style. But there actually could be a Trump effect at work. Republicans are backing most of the president’s high-profile, controversial initiatives that are popular with the GOP base, but unpopular with the general public. It could be that, with an eye toward the general election, they are compensating with more bipartisan activity behind the scenes.

The nature of the Bipartisan Index (BPI) helps explain the difference. Developed in conjunction with the McCourt School of Public Policy at Georgetown University, it scores members of Congress based on bill sponsorships and co-sponsorships, not on votes. Members score well by introducing legislation that is co-sponsored by members of the other party, and by agreeing to co-sponsor bills introduced by the other side. A score above 0.000 indicates a member is acting in a bipartisan fashion. The BPI is an objective measure, covering all substantive bills in Congress. It makes no judgement about the content of the legislation.

This GOP surge in bipartisanship meant that the average BPI score for the Senate as a whole (+.104) was above the long-term average for the first time in 10 years. The Republican Senators’ average score was +.510 on the BPI, their second highest score ever, and represents a huge jump from their +.072 score in the 114th Congress (2015-16). 

The Democrats moved in the other direction: the Democrats scored -.319 in the last Congress, down from their -.241 score in 114th Congress. The difference in scores between Senate Republicans and Democrats (.829) was the largest in the 13 Congresses covered by the Index.

In the House, the divergence between the parties was far less stark. The GOP average score was +.035, while the Democrats scored -.323. Compared to the 114th Congress, the Republicans moved from negative territory (-.152) to barely positive, while the Democrats’ score became more negative, from -.274.

Historically, the two parties have been nearly evenly matched in the Senate. Of the 13 full Congresses covered by the Index on the Senate side, Democrats have been judged more bipartisan seven times and Republicans six. 

In the Senate for the last Congress, the three Republicans from states won by Hillary Clinton (Susan Collins of Maine, Cory Gardner of Colorado and Dean Heller of Nevada) had an extremely high average BPI score of 2.062. The 37 Democrats from states won by Clinton had a negative -.557 score.

 But in Trump states, the pattern was markedly different. While the 12 Democrats from states won by Trump had, as might be expected, a strong average BPI score of +.416, the 48 Republicans from Trump states scored a nearly identical +.413. 

This apparent burst of bipartisanship by Senate Republicans comes at an unusual time, during the two years when their party held control of the White House and both Houses of Congress. A party with total control of government might feel no need to reach across the aisle, nor to accommodate proposals from the other side. It also runs counter to the narrative that Washington has become more bitterly partisan under President Trump, who routinely demonizes his opponents. Yet as Sen. Lugar noted in the release of this year’s data, there is “an undercurrent of bipartisan cooperation surrounding bill introduction.” 

The Senate GOP upswing can’t be accounted for by new faces—only two new Senators joined the Republican ranks in 2017-18, Todd Young of Indiana and John Neely Kennedy of Louisiana. It was incumbents who moved the needle. The average change for Republican Senators serving in both the 114th and 115th Congresses was an increase of 0.474 in Bipartisan Index scores. 

A small portion could be accounted for by those GOP incumbents running for re- election. Depending on circumstances, sometimes lawmakers will tack toward the partisan side with an election looming to defend against a primary challenge; or they move toward the center to improve their chances in the general election. All five Republican Senators up for re-election in 2018 raised their BPI scores-- Roger Wicker of Mississippi, Deb Fischer of Nebraska, Dean Heller of Nevada, Ted Cruz of Texas and John Barrasso of Wyoming. Heller lost in a state won by Clinton, but his opponent, Rep. Jacky Rosen, had a good BPI score in the House. Cruz faced a strong general election challenge from Rep. Beto O’Rourke who criticized him for his poor performance on the BPI. (O’Rourke scored well in the House.)

Another small factor in the jump in the average Republican Senate score was the extremely high marks achieved by Susan Collins of Maine and Rob Portman of Ohio. Their 115th Congress scores were the highest and the fifth highest, respectively, in the 26-year history of the Bipartisan Index. 

But a close look at the data shows that GOP Senate increase was driven primarily by Republican Senators signing on to bills introduced by Democrats. This could be a Trump effect. Trump is an unconventional Republican, often taking positions contrary to long-time GOP orthodoxy. Yet few Republican lawmakers have been willing to challenge him publicly on these violations. His base is loyal and enthusiastic, and the president has shown an appetite for attacking fellow Republicans who cross him. This combination could spell big trouble for a Republican in a primary election. 

On the other hand, Trump remains unpopular with the public at large. As we saw in the 2018 elections, Democrats continually challenged incumbent Republicans, “How often did you vote with Trump?” Republicans in Congress, especially Senators, face election vulnerabilities in both the primary and general election.

Most Congressional Republicans are accepting the president’s major agenda items and declining to criticize him publicly. This helps avoid a primary collision with the Trump base. But simultaneously, the Bipartisan Index shows that Republicans are quietly signing on to bipartisan legislation at a very high rate compared to what parties in the majority normally do. The motivation of each member of Congress is unique. But as a group, Republican Senators have adopted a legislative approach that has improved bipartisan activity at bill introduction and shows a clear awareness of general election vulnerabilities.