The House trended slightly in the other direction. Its overall score of -.231 was somewhat worse than the -.206 recorded in 2015-16. These changes--by +.034 and -.025 respectively--are relatively small compared to some previous years, when scores from Congress to Congress have changed by as much as .471.

Such apparent stability might not have been expected, given the GOPs unified control of the White House, House and Senate, and the tumult on Capitol Hill caused by President Trumps unorthodox governing style. One might suppose, for instance, that Republicans in total control might see no need to reach across the aisle, or that Democrats, with their increasingly liberal base, might be inclined to resist any engagement with Trump’s GOP.

But that static overall picture masks some big changes underneath, at least in the Senate. There, Democrats became significantly less bipartisan while Republicans, to a comparable degree, became more bipartisan, according to the Index.  In fact, the divergence was so sharp that the difference in scores between Senate Republicans and Democrats was the largest in the 13 Congresses covered by the Index.

Senate Democratsindex score dropped by .19 points compared to 2015-16.  (This is on a scale where the most bipartisan Senator, Susan Collins (R, ME), scored +3.15 and the least bipartisan one, Sen. Bernie Sanders, who caucuses with Democrats, ranked -2.11.)

But the big change—and perhaps big surprise—came from Senate Republicans, who increased their average score by hefty .25 points, offsetting the declines among Democrats. As a group, Republicans went from a barely bipartisan score to a creditable .323, the second-highest BPI score the caucus has registered over the past 13 Congresses.  (Historically, Senate Democrats have slightly outpaced their Republicans counterparts. Of the 12 full Congresses covered by the Index on the Senate side, Democrats have been judged more bipartisan seven times and Republicans five.) This big increase by Republicans in 2017 helped lead to the record differential between the two parties in the Senate.

Thirty-two Republican Senators earned positive scores—meaning they acted in a bipartisan fashion—while only 14 Senate Dems got positive scores. (Republicans outnumbered Democrats 52-48, although the scores dont include the Majority and Minority Leaders.) In the previous Congress, 28 GOP Senators had a positive score, when the GOP held 54 seats. (One caveat—scores for the 114th Congress cover two years, 2015-16, while we only have scores for the first year of the current 115th Congress. Scores tend to go up a bit in the second year of a Congress.)

The Senate GOP upsurge cant be accounted for by new faces—only two new Senators joined the Republican ranks last year, Todd Young of Indiana and John Neely Kennedy of Louisiana.  The average change for Republican Senators serving in both the 114th and 115th Congresses was an increase of 0.304 in Bipartisan Index scores.

We looked at Senators with the biggest swings in their BPI scores. They are hard to pigeonhole. Of the 51 Senators who improved their scores, 39 are Republicans, including all of the top 18 movers.  The three with the largest jumps in their scores were Cory Gardner of Colorado (up 1.25 points), Steve Daines of Montana (1.02) and Ben Sasse of Nebraska (1.00). All are mainstream conservatives. But the next two are Lindsey Graham of South Carolina (.92) and Rob Portman of Ohio (.90). Graham is a GOP maverick and Portman is one of the most moderate members of the caucus. Last year he catapulted into position as the second-most bipartisan Senator, behind perennial BPI champion Susan Collins of Maine, who increased her score by .60.

The 18 with the biggest improvement in their BPI scores include such conservative Republicans as Tim Scott of South Carolina and Mike Lee of Utah. Besides Collins and Portman, also among the top 18 are mainstream Republicans like Pat Roberts of Kansas and Johnny Isakson of Georgia.

(We also looked at the five incumbent Republican Senators up 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. Four of the five raised their BPI scores--Deb Fischer of Nebraska, Dean Heller of Nevada, Ted Cruz of Texas and John Barrasso of Wyoming. The other, Roger Wicker of Mississippi, showed a slight decrease in his score, but he was already one of the more bipartisan members of the Senate.)

What accounted for this surge in Republican BPI scores? According to the data, the biggest factor was Senate Republicansincreased willingness to co-sponsor bills originally introduced by Democrats.

Although its hard to say for certain, this may be a Trump effect. The Republican base is seen as solidly pro-Trump. Few Republican office holders have been willing to challenge him in public even when he veers from GOP orthodoxy. Yet because the polls say Trump remains highly unpopular among the general public, many GOP incumbents are worried about their prospects in a general election.  So they may be seeking to bolster their bipartisan resume by signing on to Democratic proposals, calculating the base wont pay much attention to legislation they co-sponsor, as long as they dont criticize or attack the president.

A different dynamic may be at work on the Democratic side, where 25 incumbents are running for re-election. Ten of them saw their BPI scores rise last year, compared to 2015-2016, accounting for nearly all of the Democratic Senators with higher scores.  All but two of the rest, whether or not facing races this year, had lower bipartisan scores. (The two exceptions: Gary Peters of Michigan, whose score rose .36, to make him the eighth most bipartisan Democrat, and Jeff Merkley of Oregon, whose score edged up a miniscule .004, leaving him in position as among the least bipartisan.)

The Democratic Senator with the biggest improvement was liberal firebrand Elizabeth Warren of Massachusetts. Not only is she up for re-election this fall, but she is widely believed to be angling for a possible run at the presidency. Despite her .45 increase, she remained in the bottom half of the Senate for bipartisanship.

The others with both races and an increase in scores include moderates who already rank high on bipartisanship, like Claire McCaskill of Missouri, Angus King of Maine and Heidi Heitkamp of North Dakota; liberals who rank low but improved somewhat--Tammy Baldwin of Wisconsin, Mazie Hirono of Hawaii, Bob Menendez of New Jersey and Sherrod Brown of Ohio; and two in the middle, Martin Heinrich of New Mexico, whose score rose significantly to a negative .18, and Maria Cantwell of Washington, whose score edged up a hair to .18. (By far the biggest swing among Democratic Senators was Tom Carper of Delaware, who faces re-election and went from solidly bipartisan (.53) to solidly not bipartisan (-.52).

In other words, without the boost from Senators seeking re-election, the Democratic BPI score for 2017 would have fallen even farther. Most Democrats have apparently concluded that President Trump is so unpopular with both their base, which has become highly motivated, and with the general population that there is little to be gained from cooperating with the GOP. Many incumbents facing the voters this cycle--but clearly not all--may be feeling the need to polish their bipartisan bona fides in an uncertain political climate.

A Trump effect may be evident in another way. Of the 47 Senate Democrats ranked in the Index for 2017, the bottom 22 all come from states won by Hillary Clinton in the 2016 presidential race. By contrast, Democratic Senators from Trump states, who are considered more at risk, are bunched toward the top of the BPI rankings for Senate Democrats, indicating they are working harder at bipartisanship than many of their party colleagues. 

Sherrod Brown, a liberal stalwart from Ohio, and Debbie Stabenow of Michigan, were the two lowest-ranked Democratic Senators from a Trump state (#19 and #25 among Democrats, respectively.)  They took different tacks—Brown essentially held steady, while Stabenow veered away from bipartisanship, lowering her score by .46 to wind up deep in partisan territory.

On the House side, the divergence between the parties last year was not as great. The range of scores is narrower, from +2.09 to -1.81, than in the Senate. The House as a whole became less bipartisan--its overall score dropping from -.206 to -.231--and in contrast to the Senate, both parties had negative scores. The House GOP score of -.142 in the 115th Congress was about the same, a .01 improvement from the 114th. The Democrats started out and remained even less bipartisan (-.340), their scores having dropped by .066.

There may be several reasons for the difference in House and Senate behavior last year. For one, all the House incumbents, both Republicans and Democrats, have survived a Trumpian election, so theyve already tested their survival skills and positioned themselves appropriately for their districts. By contrast, only a third of the Senate overall, and none of the incumbents running this year, were on the ballot for the Trump-Clinton race.

For another, thanks to gerrymandering and other factors, most House districts are solidly red or solidly blue. Few House members--but perhaps more this year than previously--face stiff primary or general election challenges. They may not feel the need to tack left or right for re-election purposes. On the other hand, the unusually high number of announced retirements by prominent House members may indicate that they feel altering their political posture for the new environment is unnecessary.