Donald Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE’s biggest hurdles next year might not come from Democrats but from within his own party.

With a narrow 52-seat majority in the Senate, the president-elect will need to keep Republicans unified if he wants to clear nominations and legislation through the upper chamber in the face of likely Democratic opposition.

Many Senate Republicans tepidly embraced Trump during the campaign, and a handful refused to endorse him altogether. Trump’s ability to get his agenda cleared through Congress will largely depend on whether he can get some of his biggest critics on his side.

Here are five Republicans who could buck Trump in 2017:

Sen. Susan Collins Susan Margaret CollinsClub for Growth to spend million in ads for Trump Supreme Court nominee Maryland's GOP governor says Republicans shouldn't rush SCOTUS vote before election The Hill's Morning Report - Sponsored by Facebook - GOP closes ranks to fill SCOTUS vacancy by November MORE (R-Maine): Collins is one of the caucuses’ more moderate Republicans and will be crucial for getting Trump’s agenda through the upper chamber.

The Maine Republican didn’t support Trump’s campaign, coming out early for former Florida Gov. Jeb Bush.

She signaled for months that was uncomfortable with Trump’s campaign rhetoric and announced in August that she couldn’t support the real estate mogul because he “does not reflect historical Republican values nor the inclusive approach to governing that is critical to healing the divisions.”

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Since then, Collins has signaled that she’s willing to work with Trump on areas of common agreement—like infrastructure spending—but also told a local TV station that the real estate mogul has an “obligation” to show that he’s moved past his controversial remarks.

But she’s also continued to break with Trump on a number of policy issues. She told the Portland Press-Herald that she was “concerned” with Trump dismissing reports that Russia meddled in the U.S. elections.

She separately told the paper that she also has “reservations” about a possible push to privatize Medicare and is undecided on whether to repeal the Affordable Care Act without a replacement plan.

Republicans are planning to use the reconciliation process to repeal large parts of ObamaCare, meaning they can only afford to lose up to three GOP senators.

Sen. John McCain John Sidney McCainThe Memo: Trump's strengths complicate election picture Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' Cindy McCain: Trump allegedly calling war dead 'losers' was 'pretty much' last straw before Biden endorsement MORE (R-Ariz.): The Arizona Republican appears most likely to break with Trump on foreign policy and the military, where McCain has substantial sway as Armed Services Committee chairman.

The two were at odds from nearly the start of the presidential campaign, when Trump mocked McCain’s military service and years as a prisoner of war saying “I like people who weren’t captured.”

McCain formally dropped his support for the real estate mogul in early October and has refused to answer questions about Trump for months—a policy he says he’ll change come Jan. 1.

The two repeatedly clashed over the issue of torture as Trump floated bringing back interrogation methods like waterboarding.

McCain doubled down after the election, saying at the Halifax International Security Forum that: “I don’t give a damn what the president of the United States wants to do. We will not torture people.”

They are also already on opposing sides over Russia, with McCain leading the push for a select committee to investigate the CIA’s finding that Russia intervened in the election to help Trump.

Trump has dismissed those claims, arguing that Democrats are grasping for reasons to explain why they lost the presidential race.

McCain is likely to be joined in the fight by Sen. Lindsey Graham Lindsey Olin GrahamSenate GOP aims to confirm Trump court pick by Oct. 29: report The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Senate GOP sees early Supreme Court vote as political booster shot MORE, his closest Senate ally. Speaking to dozens of reporters after the election, the South Carolina senator warned he would be a “hard ass” on Moscow.

Sen. Mitch McConnell Addison (Mitch) Mitchell McConnellFEC flags McConnell campaign over suspected accounting errors Poll: 59 percent think president elected in November should name next Supreme Court justice Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' MORE (R-Ky.): The Senate’s top Republican largely avoided weighing in on Trump’s slew of controversies, regularly declining to answer questions about his party’s standard bearer in 2016.

But McConnell made headlines during his book tour for sharply criticizing the GOP nominee and denounced his 2005 comments about groping and kissing women without their consent. McConnell quickly demanded the real estate mogul take “full responsibility” for the leaked remarks.

"As the father of three daughters, I strongly believe that Trump needs to apologize directly to women and girls everywhere, and take full responsibility for the utter lack of respect for women shown in his comments on that tape," he said at the time.

McConnell—like the rest of GOP leadership—is projecting unity with Trump, eager to prove that Republicans can govern as they control both Congress and the White House for the first time in a decade.

But as the Senate’s top lawmaker he also has the ability to effectively kill any proposal he disagrees with. He’s pushed back against two of Trump’s more controversial campaign proposals—banning Muslims from entering the country and building a border wall at Mexico's expense.

He also reiterated his support for not criminalizing flag burning after Trump floated revoking the citizenship of flag burners. McConnell's also laying down early goalposts as Trump eyes deep tax cuts.

“I think this level of national debt is dangerous and unacceptable,” he told reporters. “My preference on tax reform is that it be revenue neutral,” he said.

Sen. Rand Paul Randal (Rand) Howard PaulRand Paul says he can't judge 'guilt or innocence' in Breonna Taylor case Overnight Health Care: Health officials tell public to trust in science | Despair at CDC under Trump influence | A new vaccine phase 3 trial starts Health officials tell public to trust in science MORE (R-Ky.): Paul ultimately backed Trump during the campaign—after ending his own failed White House bid—but he’s drawing early lines over some of the real estate moguls’ nominees.

Paul, a member of the Foreign Relations Committee, played hardball on the State Department, and is now threatening to block former U.N. ambassador John Bolton from taking the No. 2 spot.

Paul told ABC’s “This Week” that he is an “automatic no on Bolton,” who he called a “big cheerleader for the Iraq War” with a “naive understanding of the world.”

Paul’s opposition would be enough to block Bolton from getting through the Foreign Relations Committee if every Democrat also votes against him.

He also took aim the rest of Trump’s picks in his annual “Festivus” tweetstorm, quipping: “New administration is lookin good. Haven't seen this many billionaires in 1 place since I staked out Bilderbergs w/ Alex Jones. Good times."

The Kentucky Republican is also warning he won’t vote for a budget that doesn’t balance within five years.

Paul isn’t the only GOP senator who regularly votes against budget and funding bills. GOP Sen. Mike Lee Michael (Mike) Shumway LeeMcConnell shores up GOP support for coronavirus package McConnell tries to unify GOP Davis: The Hall of Shame for GOP senators who remain silent on Donald Trump MORE (Utah) also frequently votes against the money bills.

Sen. Ben Sasse (R-Neb.): The Nebraska Republican has been a perennial critic of Trump, frequently comparing the 2016 White House race to a “dumpster fire” and spending months voicing concerns about his party’s standard bearer.

In a Facebook letter directed at Trump supporters, he warned that the real estate mogul was too divisive and could ignore constitutional checks and balances aimed at preventing executive overreach.

“Have you noticed how Mr. Trump uses the word ‘Reign’ – like he thinks he’s running for King? It’s creepy, actually. Nebraskans are not looking for a king,” he said during the GOP presidential primary.

Sasse—whose criticism of Trump is frequently delivered in evening tweetstorms—has signaled he’s willing to work with the incoming administration in some areas, like filling the Supreme Court vacancy, without caving on other issues.

He wrote in an Omaha World Herald op-ed after the election that while he’s seen positive signs from Trump since his White House win there “are absolutely some things that worry me.”

“There will be disagreements — between neighbors, between the executive and legislative branches, between political parties. This is a good thing. This is an intentional feature of our system, not a bug,” he added.