After they install a ninth Supreme Court justice, the GOP will control all three branches of government. So it stands to reason that whoever plays a pivotal role in blocking President Trump would be Republican, too.

In some cases, all it will take is a few Republicans in Congress to derail his agenda, especially when it comes to the Senate approving his appointees in the critical first few days of his presidency. Scroll down to the bottom for the math*, but basically Democrats would need to win over just three Republicans to block Trump’s appointees. There are at least eight Senate Republicans who ditched Trump during the primaries.

Ten days after Trump’s election, the cracks are already starting to show between Trump and key congressional Republicans on Trump priorities like immigration, term limits and some of the president-elect’s top advisers.

Here’s a list of six Republicans poised to make Trump’s life very difficult, should they choose:

1. Sen. Rand Paul (Ky.)

(Patrick Fallon/Bloomberg)

Why he could make Trump’s life difficult: Paul, who ran for president himself, is one of the Senate’s most prominent non-interventionists. In other words, he and other libertarian-leaning politicians want to steer clear of most international conflicts, which is generally in line with the foreign policy views Trump espoused on the trail.

But then word floated that Trump was considering one of the most hawkish hawks in foreign policy, John Bolton (a Bush administration ambassador to the United Nations), to be secretary of state. Former New York City mayor Rudy Giuliani’s name is also being floated for the job. Paul isn’t a fan of either man.

What he’s said: “I can’t support anybody to be our secretary of state who didn’t learn the lesson of the Iraq War,” he told Reason Magazine.

“I’ll do whatever it takes to stop someone like John Bolton being secretary of state,” he told Politico.

“Will Donald Trump betray voters by hiring John Bolton?” he asked in an op-ed.

But: Paul also told Politico he’s more inclined to “start a conversation” than actually try to block any of these nominees.

2. Sen. Lindsey Graham (S.C.)

(J. Scott Applewhite/AP)

Why he could make Trump’s life difficult: Graham was one of Trump’s most vocal GOP critics during the campaign. (Graham also briefly ran against Trump for president.) Graham didn’t even vote for Trump.

Also, Graham wants to investigate Russia — specifically, whether it hacked into Democrats’ emails during the campaign. His call for an investigation came amid news Trump and Russian President Vladimir Putin chatted on the phone after the election and both agreed to improve relations. Trump alarmed Graham and many other foreign policy intellectuals when he declined to criticize Putin during the campaign.

Graham — a hawk and respected defense leader on the Hill — said he’s encouraged by Trump’s apparent willingness to boost defense spending. But he also told the Military Times he wouldn’t support some of Trump’s immigration policies.

What he said: “Clearly me and the Donald have issues, and I will do everything I can to help him because he will be commander in chief in dangerous times,” he told reporters on Capitol Hill this week. “I worry about Russia.”

But: Graham is trying to play nice with Trump for now.

3. Sen. John McCain (Ariz.)

(Chip Somodevilla/Getty Images)

Why he could make Trump’s life difficult: For pretty much the same reasons Graham could. McCain broke with Trump during the primary and he’s someone else who is alarmed by Trump’s apparent embrace of Russia. (Coincidence? McCain is also the third senator on our list to have run for president, although McCain didn’t run against Trump.)

What he said: Here’s a statement McCain released Tuesday. It’s heavily critical of Trump’s initial approach to Russia:

“With the U.S. presidential transition underway, [Russian President] Vladimir Putin has said in recent days that he wants to improve relations with the United States. We should place as much faith in such statements as any other made by a former KGB agent who has plunged his country into tyranny, murdered his political opponents, invaded his neighbors, threatened America’s allies, and attempted to undermine America’s elections.”

But: McCain, like Graham, hasn’t outlined any specific ways he’d block Trump — on Russia or anything else.

4. Sen. Susan Collins (Maine)

(J. Scott Applewhite/AP)

Why she could make Trump’s life difficult: Collins is one of the most moderate Republicans — if not one of the only true moderates — in the Senate. During the campaign, she broke with Trump early on, calling him unsuitable for office in an August op-ed for The Washington Post. On Election Day, she wrote in House Speaker Paul Ryan (R-Wis.) for president.

What she’s said: She congratulated Trump on his win and told WABI Channel 5 in Maine she’s hopeful that Trump will strike a more conciliatory note as president: “[S]ome of what Donald Trump said is totally unacceptable, and I was very critical of some of what he said, and I think there is an obligation on him to show that he’s leaving that behind.”

Collins, who is up for reelection in 2020 (or could run for governor of Maine in 2018), is also under some pressure back home to continue to speak out against Trump.

In an editorial Tuesday, the Portland Press-Herald called on her to denounce Trump’s choice of former Breitbart News chief Steve Bannon as his top aide: “We hope Collins, who publicly declared Trump unacceptable during the campaign, will continue to act as a moderating force in a federal government soon to be completely under the control of Republicans.”

But: Unlike most of the other candidates on this list, so far Collins hasn’t said or done much to indicate she’s planning to be a major thorn in Trump’s side.

5. Sen. Ben Sasse (Neb.)

(Bill Clark/CQ Roll Call via AP)

Why he could make Trump’s life difficult: The freshman senator is the Senate’s original #NeverTrump-er. “I’m as frustrated and saddened as you are about what’s happening to our country,” he wrote in a Facebook post in February. “But I cannot support Donald Trump.” He voted for Mike Pence for president.

What he’s said: Honestly, based just on what he’s said since the election, Sasse is the least likely senator on this list to make life difficult for Trump. He wrote an op-ed in the Omaha World Herald saying he’ll look for places to compromise with Trump, though he encouraged voters to continue to be critics when necessary.

But: Sasse also surprised us by criticizing the media Thursday for not looking into “paid riots” (not a thing) re the protests going on in cities across the nation against Trump. It was a tweet we would expect to come from the president-elect’s account, not the Senate’s original Never-Trump-er.

6. Sen. Jeff Flake (Ariz.)

(Andrew Harnik/AP)

Why he could make Trump’s life difficult: During the campaign, Flake also earned a reputation as one of Trump’s most vocal critics in Congress. “I don’t think that that’s our best foot forward at all,” Flake told reporters this spring. In July, he literally stood up to Trump in a closed-door Capitol Hill meeting with the nominee.

What he’s said: Now that Trump is going to be president, Flake is playing nice, too. In an interview Wednesday with CBS News, he declined to criticize Trump’s choice of Bannon for a top aide.

“I’m of the mind to give the president a lot of deference when he’s putting together his team,” he said.

But: Flake stays on this list because there are several flash points that could pop up between him and Trump: Molly Reynolds with the Brookings Institution pointed out that Flake is particularly opposed to increased federal spending, and Trump’s infrastructure plan would cost $500 billion.

*Thanks for reading this far! Here’s our math breakdown on why it will take just three senators to block some of Trump’s appointments.

You only need 50 senators to approve Trump nominations. (That’s thanks to Senate Democrats removing the option of a filibuster for most political and judicial appointees. And Vice President-elect Mike Pence (R) can serve as tie-breaker.) Come 2017, there will be 52 Republicans in the Senate and 48 Democrats. (Louisiana’s Senate race is outstanding pending a December runoff, but all indications are Republican John Kennedy will win.)

So 52 Republican senators – 3 Republican senators + 0 Democratic senators = 49 senators — in other words, not enough to get a nominee over the line.