Republicans will control both sides of the Capitol next year, and with Donald Trump in the White House, GOP lawmakers are eager to make good on their campaign promises and enact sweeping changes to everything from Obamacare to immigration.

But there's a catch: Their narrow 52-vote majority in the Senate doesn't give Republicans a lot of margin for error. They'll have to win some Democratic support if they want to block any filibuster threats, and red-state senators up for reelection in 2018 are likely to be prime targets. And even a small number of GOP defections — particularly from the party's dwindling moderate wing — could spell an early demise for overly ambitious proposals.


Even when it comes to nominations, most of which can win Senate confirmation with a simple majority vote, Trump may have to mount an uncharacteristic charm offensive to keep Republicans united or enlist Democrats to back any particularly polarizing selections.

“In the past, when Republicans were challenging President Obama, it was kind of a categorical condemnation of everything,” Sen. Bob Casey (D-Pa.) said in a brief interview last week. “I don’t think we’ll do that. But we’re going to have some real fights.”

So with an eye to when those fights start in earnest next year, here's a look at the most powerful swing votes in the Senate in the next Congress — a combination of red-state Democrats on the ballot in 2018 and bluish-state Republicans who survived 2016.

Joe Manchin (D-W.Va.)

Manchin could easily be paired with fellow red-state Sens. Heidi Heitkamp (D-N.D.) and Joe Donnelly (D-Ind.) as a swing-voting bloc on this list. But among the 10 Senate Democrats facing reelection in 2018 in states that Trump turned red, Manchin faces the hardest road — the president-elect won West Virginia by 42 points. Manchin opposed the majority of his party on floor votes more often than any other Democrat so far this year, according to a POLITICO analysis, with Heitkamp and Donnelly right behind him when it comes to splitting with the Democratic caucus.

What’s more, Manchin’s new position in Democratic leadership gives him a high-profile platform to nudge incoming Minority Leader Chuck Schumer (D-N.Y.) toward compromise with Trump and the GOP. His early support for Sen. Jeff Sessions’ attorney general nomination helped ensure the Alabama Republican’s apparent glide path to confirmation.

“I want the place to work,” Manchin told reporters last week. He’s expected to align with Republicans on several early Trump priorities, including infrastructure and expanding domestic fossil-fuel production. On other thorny issues on which the GOP may lose a small number of votes from its own ranks, such as immigration and Obamacare, Manchin is a likely but less guaranteed get.

Rand Paul (R-Ky.)

The libertarian and critic of what he perceives as excessive governmental power has already delivered Trump the senatorial equivalent of a brushback pitch, vowing early last week to oppose John Bolton or Rudy Giuliani if the president-elect taps either man as secretary of state. Paul later predicted that there are “several potential Republican votes” against either choice, forecasting “a very close vote.”

That won’t be the last time Paul, who often breaks from his party on foreign policy and electronic surveillance issues, makes trouble for Trump even after supporting him during his campaign. If Trump proposes to pay for the massive infrastructure package he’s proposed with new federal spending, Paul is likely to be on the vanguard of conservative resistance. Paul later vowed to press Trump’s nominee for CIA director, Rep. Mike Pompeo (R-Kan.), over his support for the PATRIOT Act and expanded authority for the National Security Agency.

Pompeo is “going to have to also answer to my liking whether or not he's still for torture, whether or not he's for waterboarding," Paul said during a Nov. 20 appearance on “Face the Nation.”

Dean Heller (R-Nev.)

In an otherwise punishing 2018 map for Democrats, Nevada is the party’s best and perhaps only opportunity to take a Senate seat from the GOP. And Democrats have reason to be optimistic about their chances of toppling Heller, thanks to Sen.-elect Catherine Cortez Masto’s (D-Nev.) victory in a hotly contested race and their successful recapture of both houses of the state legislature.

Because term limits will stop Nevada’s popular Republican Gov. Brian Sandoval from running for reelection, Heller is occasionally talked about as an odds-on favorite to replace him. That makes the genial 56-year-old, who kept his election-season skepticism of Trump at a much lower volume than some of his colleagues, a low-key Senate swing vote to watch.

“My job is to go back there and work with the new president,” Heller told Nevada’s KLAS TV after the election. “If I like his policies, I’ll support them.”

Look for Heller to get pressure to break from Trump and fellow Republicans on immigration, given that strong Latino turnout has helped Democrats take Nevada in the past three presidential elections. He’s also likely to fight back if Trump seeks to resuscitate the proposed nuclear waste disposal site at Nevada’s Yucca Mountain, an issue the president-elect sidestepped during the campaign.

Jon Tester (D-Mont.)

As he eyes a reelection bid in a state Trump won by 21 points, Tester can take some consolation in Montanans’ decision to give another term to Democratic Gov. Steve Bullock. But with Rep. Ryan Zinke (R-Mont.) openly eyeing a challenge and urging Tester to move to the middle, the former Democratic Senatorial Campaign Committee chairman could give his party fresh heartburn this year by working with the GOP.

Tester recently told KPAX TV in Montana that lawmakers should “start working together” after the bitterly fought election and signaled he’s open to revisiting elements of Obamacare but not scrapping it. He also suggested that Trump’s rhetoric on infrastructure would find Democratic support.

Susan Collins (R-Maine)

Collins distanced herself from Trump back in August, publishing an op-ed that lamented his “constant stream of cruel comments and his inability to admit error or apologize.” She later told local media that she cast a write-in vote for Speaker Paul Ryan (R-Wis.). But now that Trump and Ryan are striking a chord of unity — for the moment — Collins will have to decide how closely to align with her party after crossing the aisle on more than a dozen floor votes so far this year.

Collins, who lost to junior Maine Sen. Angus King, an independent, in the 1994 governor’s race, is also considered a prospective candidate for governor in 2018. Collins’ reputation for bipartisan collaboration could prove a major selling point if she mounts a run to replace term-limited Republican Gov. Paul LePage, whose approval ratings are dismal.

Collins came out in support of Sessions’ attorney general nomination hours after its release, suggesting that she may be inclined to give Trump leeway on nominations after urging her own party to hold hearings for President Barack Obama’s Supreme Court nominee. Look for her to support changes to Obamacare provided that its most popular elements are preserved, such as the prohibition on denial of insurance coverage to people with preexisting conditions that Trump already has said he wants to maintain.