After Democrats and Republicans reached record highs sticking together by party on congressional votes in 2017, those numbers nose-dived in 2018 as lawmakers worked across the aisle on high-profile legislation, including a rewrite of the Dodd-Frank financial law, a package dealing with the opioid crisis, spending bills and an overhaul of the country’s criminal justice laws.

CQ’s annual vote study shows that in the House the total number of party unity votes — defined as those with each party’s majority on opposing sides — fell from 76 percent of the total votes taken in the House in 2017, a record, to 59 percent in 2018. That latter figure is the lowest since 2010, the most recent year of unified Democratic control of Congress. Election years typically have fewer votes and 2018 was no exception — the total number of votes taken in the House, 498, was the lowest since 2002.

[Download the CQ Vote Studies-Party Unity data and graphics]

In the Senate, the decline was even more dramatic — the total number of unity votes dropped 19 points from the year before — from 68.9 percent of all votes taken down to just under half of all votes — 49.6 percent. That marks the second-lowest figure since 2002.

Republican Sen. John Cornyn of Texas seemingly predicted such an occurrence. He said last year that he thought 2018 would be different because “the margin is thinner, and thankfully there are a number of things we agree on.” He noted a desire among lawmakers to work on overhauls to financial regulation and criminal justice, two big-ticket items that got signed into law in 2018 with bipartisan support.

Molly E. Reynolds, a fellow at the Brookings Institution, agrees, saying that in 2017, “We saw the Senate controlled by the Republicans using a lot of its agenda to pursue things on which they didn’t need Democrats.” These votes were generally in three distinct buckets: nominations, Congressional Review Act resolutions striking down Obama-era regulations, and the budget reconciliation process for a successful overhaul of the tax code and a failed attempt at repealing the Affordable Care Act. None of those votes were subject to the 60-vote filibuster threshold, and nearly all Republicans and Democrats held together on many of these votes, upping their unity scores.

In 2018, things changed, as is typical of unified congressional control in an election year, says Reynolds, as lawmakers seek to show voters they can govern. A host of must-pass bills needed bipartisan support to get through Congress, including a reauthorization of the Federal Aviation Administration, the farm bill and a water infrastructure bill.

“In year two we still see a fair amount of nomination-related votes, but there’s many more things that the Senate was considering in the second year that it needed 60 votes to get done,” she said.

Still, the 2018 election succeeded in wiping out a large swath of moderates on both sides of the aisle — those members who most often crossed their party on such votes — making it an open question just how much legislating will get done in the next two years.

The early results for 2019 suggest that unity scores will return to their 2017 levels: Most legislation that has moved in the Democratic-controlled House has broken along partisan lines. The Republican-controlled Senate has been a tad more bipartisan. And if history is any guide, a divided Congress during the last two years of a president’s first term usually means a sharp dive in enacted laws.

Watch: A behind-the-scenes look at CQ’s 2018 Vote Studies

The likelihood, say observers, is that Democrats in the House will pass partisan bills they campaigned on, such as an ethics and campaign finance bill that has no prospects in the Senate.

Even in this barren environment, there are some opportunities to work on meaningful legislation, Reynolds noted. Two high-profile chairmen in Congress have already talked about ways to combat high prescription drug prices: GOP Sen. Charles E. Grassley of Iowa, the Finance Committee chairman, and Democratic Rep. Frank Pallone Jr. of New Jersey, the House Energy and Commerce chairman. Already, hearings have been held in both chambers on the matter. Republican allies of President Donald Trump say the president is serious about working on a solution in a bipartisan way. Still, said Reynolds: “I’m not terribly optimistic.”

In addition, Trump’s trade policies will continue to come under scrutiny. Grassley has been a vocal opponent of administration-imposed tariffs on aluminum and steel, as has the new chairman of the House Ways and Means Committee, Democrat Richard E. Neal of Massachusetts.

Congress will also need to make a decision on Trump’s renegotiated North American Free Trade Agreement.

The chances for any overhaul to immigration are small following Trump’s emergency declaration on building the wall along the southern border.

Despite the fall in total unity votes, both Democrats and Republicans maintained fealty to their parties overall in 2018. In the House, Republicans stuck with their party an average of 91 percent of the time on such votes and Democrats did 89 percent of the time, both slightly down from 2017, but still hovering around historic highs.

The Republican position prevailed on 92.1 percent of the 269 House votes that split majorities of the two parties. Such a feat can be largely attributed to tight control by party leaders on how legislation reaches the floor: In the 115th Congress, the number of bills that reached the floor without any chance for amendment — so-called closed rules — broke the previous record of 83 set four years ago.

In the Senate, Republicans worked with a narrower majority in 2018, as Democrat Doug Jones of Alabama won a special election to serve out the remainder of Republican Jeff Sessions’ term, and Republican John McCain of Arizona was away from Congress the entire year until his death in August.  The result was a 50-49 split and a decline in wins for Republicans on party unity votes — from 89.7 in 2017 to 79.4 in 2018 on 136 votes.

Still, even with that decline, the win percentage for Republicans was the second-highest since 2005. The figure was bolstered by Vice President Mike Pence, who was brought in to break ties in the Senate seven times, up from six times in 2017.

In the Senate, five Democrats had perfect unity scores, all but one of them either declared or potential 2020 presidential aspirants: Cory Booker of New Jersey, Kirsten Gillibrand of New York, Jeff Merkley of Oregon and Elizabeth Warren of Massachusetts. Edward J. Markey of Massachusetts was the exception.

Those senators will be closely watched on votes this year. They will be working on a dual track to court primary votes in what is proving to be an already crowded race for the Democratic nomination, while needing to move must-pass legislation like spending bills and an impending debt ceiling hike. In 2017, Gillibrand made an early decision to oppose nearly every nomination to Trump’s Cabinet, and she continued that opposition in 2018, helping push her unity score to a perfect 100 percent.

Reynolds sees these Democratic senators playing a crucial role in 2019, and foresees Majority Leader Mitch McConnell scheduling a host of potentially tough votes. Already, he has begun making them walk the plank on a bill scheduled for a vote Feb. 25 that would require that babies born during a botched abortion be given the same care as any other human. He is also planning to hold a vote on the so-called Green New Deal, a Democratic proposal dealing with climate change and economics.

“McConnell wants to get those senators on record,” said Reynolds, the Brookings scholar. “That could cut both ways. There are some senators who are running for president who are eager to register their support for something like that. I think there are other ones who are a little more nervous.”

The statistics compiled by CQ show that both Democrats and Republicans shed lawmakers in the 2018 election who most often crossed over to vote with the other party on votes.

In the Senate, a group of red-state Democrats often joined with Republicans on votes for federal judges and the rewrite of the Dodd-Frank bill.

But of the top six Democrats who broke from their party the most in 2018, four are no longer in Congress, having been defeated by a Republican challenger: Heidi Heitkamp of North Dakota, Joe Donnelly of Indiana, Claire McCaskill of Missouri and Bill Nelson of Florida.

The only two of that group who remain are Joe Manchin III of West Virginia and Doug Jones of Alabama. Jones, who is up for election to a full six-year term in 2020, has already cast votes that Republicans are poised to use against him for re-election, including a “no” in September in the confirmation of Supreme Court Justice Brett M. Kavanaugh. The Alabama GOP was quick to say that Jones’ vote “betrays our state.”

In the House, it was Republican moderates who got wiped out: Of the top 11 Republicans crossing over to vote with the other party the most in 2018, seven are no longer in Congress. Three lost re-election: Leonard Lance of New Jersey, Mark Sanford of South Carolina and Carlos Curbelo of Florida. Three retired: Ileana Ros-Lehtinen of Florida, Frank A. LoBiondo of New Jersey and Ryan A. Costello of Pennsylvania. The one who broke with his party the most, Walter B. Jones of North Carolina, died in February.

The other four from that group who remain in Congress are Justin Amash of Michigan, Brian Fitzpatrick of Pennsylvania, Thomas Massie of Kentucky and Christopher H. Smith of New Jersey.

“Certainly centrists get squeezed out when there’s a wave,” said Curbelo, a moderate Republican from Florida who introduced a carbon tax bill in 2018 but lost his re-election bid in 2018. He questions whether the two-party system is viable anymore.

“The two-party system is yielding a lot of leaders who don’t see the incentive to compromise, don’t see the incentive to negotiate and actually solve the nation’s problems,” he said. “On the contrary, they like to leave the challenges unsolved. That way they can exploit them come campaign season.”

While it’s true that Republican moderates got the axe in 2018, many Democratic moderates held on, including the top 10 who broke with the party the most on unity votes: Kyrsten Sinema of Arizona, who won her election bid for Senate; Henry Cuellarof Texas; Collin C. Peterson of Minnesota; Josh Gottheimer of New Jersey; Stephanie Murphy of Florida; Brad Schneider of Illinois; Conor Lamb of Pennsylvania; Jim Costa of California; Jim Cooper of Tennessee; and Tom O’Halleran of Arizona.

A number of names on that list, like Murphy, O’Halleran and Peterson, were targeted by Republicans in 2018, but they survived nevertheless. The first list released by the National Republican Congressional Committee earlier this month includes all three again this year, and 52 others, the majority freshman. 

In a release targeting the Democrats, the NRCC made it clear how they are going to go after them in 2020, by holding “these targeted members accountable for the radical policies being pushed by the socialist Democrats in their party.”

Guide to the Vote Studies

CQ has analyzed voting patterns of members of Congress since 1945. The three current studies — presidential support, party unity and voting participation — have been conducted in a consistent manner since 1954.

Selecting votes: CQ bases its vote studies on all floor votes for which senators and House members were asked to vote “yea” or “nay.” In 2018, there were 498 such roll-call votes in the House and 274 in the Senate. The House total excludes two quorum calls in 2018.

The House total counts all votes on procedural matters, including votes to approve the journal.

Individual scores: Member scores are based only on the votes each actually cast. This makes individual support and opposition scores total 100 percent. The same method is used to identify the leading scorers.

Overall scores: To be consistent with previous years, calculations of average scores by chamber and party are based on all eligible votes, whether or not all members cast a “yea” or “nay.” The lack of participation by lawmakers in a roll call vote reduces chamber and party average support and opposition scores.

Rounding scores: In the tables, scores are rounded to the nearest percentage point. Scores for the presidential and party support leaders are reported to one decimal point in order to rank them more precisely.

[Trump’s winning pattern with legislation might become a thing of the past: CQ Vote Studies]

[Key Senate votes in 2018: CQ Vote Studies]

[Key House votes in 2018: CQ Vote Studies]

[At least senators showed up to vote in 2018: CQ Vote Studies]

This story originally appeared in CQ Magazine’s “Annual Vote Studies & 2018 Key Votes” special report on Feb. 25.