Donald Trump sits atop Republicans in Washington, but that doesn’t necessarily mean the Senate will rubber-stamp everyone he picks for his Cabinet.

While he can expect the Republican Senate to confirm the vast majority of his choices, the party’s narrow 52-48 edge and early infighting could slow or doom controversial ones.

Sen. Rand Paul of Kentucky was the first out of the gate putting Trump on notice Tuesday, warning against picking former Ambassador John Bolton or Rudy Giuliani as secretary of State.

“Heaven forbid,” he wrote about Bolton, blasting him as someone who “never learned and never will” that the Iraq war was a mistake, and who advocated a war against Libya that Trump has since opposed as well as other neoconservative adventures.

“The man is a menace,” Paul wrote.

He also told the Washington Post and CNN he isn’t inclined to back Giuliani either.

And given Paul’s perch on the Senate Foreign Relations Committee, he could team up with Democrats to block a pick. The panel has a narrow 10-9 Republican majority, meaning a single senator could join with the panel’s Democrats to block a nomination. The same could happen in other committees.

On the Senate floor, just three Republicans could block a Trump nominee provided that Democrats stay united.

Sen. John Cornyn of Texas, the Republican whip, said it makes sense for the administration to talk to senators before making decisions.

“I think it would be good to collaborate,” he said.

There are a number of Republican senators who either never endorsed Trump or unendorsed him — from conservatives like Mike Lee of Utah and Ben Sasse of Nebraska to more moderate members like John McCain and Jeff Flake of Arizona, Lisa Murkowski and Dan Sullivan of Alaska, Lindsey Graham of South Carolina, Cory Gardner of Colorado, Susan Collins of Maine and Rob Portman of Ohio.

Collins said in an interview she’s inclined to back the president’s team but would wait to see the nominees.

“This is the president-elect’s choice, and he should make up his own mind,” she said. “I’m sure he’s doing some consultations but that’s really up to him.”

A few other senators, like John Thune of South Dakota and Deb Fischer of Nebraska, had called on Trump to step aside after the Access Hollywood tape aired, but jumped back on the Trump train when it became clear he wouldn’t.

And then there’s Ted Cruz of Texas, who only endorsed Trump late in the campaign after getting booed off the stage at the Republican National Convention for telling people to vote their “conscience.”

They’ve all since talked about working with a Trump administration. On Tuesday, for example, Graham told reporters that he could support Bolton or Giuliani for secretary of state.

But there could still be individual appointments that cause trouble. Take immigration, where senators like Flake and Graham have opposed Trump’s threats of mass deportation.

“I will not vote for a bill that treats a grandmother and a drug dealer the same,” Graham said Tuesday. Given the wide discretion that Department of Homeland Security appointees have over deportations, Trump’s picks there could get extra attention.

The same goes for intelligence and national security positions. McCain has criticized Trump’s remarks on torture. During the campaign, Trump blessed waterboarding and advocated even harsher measures.

Other senators, including Cruz, have been critical of National Security Agency eavesdropping programs and could make stands on those choices. Cruz and Lee both sit on the Armed Services Committee, which has jurisdiction over some of the relevant agencies.

Trump will also have to keep Mitch McConnell happy; the majority leader determines when — or if — nominations get votes. Trump, Vice President-elect Mike Pence and Alabama Senator Jeff Sessions, a key Trump ally, are among the people who have made the trek to the majority leader’s office so far.

Sessions has been mentioned as a possible secretary of defense or even attorney general. Even though he is a Republican senator, he could face some scrutiny too. Back in 1986, the Senate blocked his nomination for a federal judgeship after he was accused of making racist remarks. A confirmation hearing for a Trump cabinet post would certainly resurface those allegations.

Trump does have one big advantage, and he can thank Harry Reid of Nevada. The retiring Democratic leader’s use of a maneuver known as the “nuclear option” to change Senate rules to prevent a minority from blocking nominees now means Democrats only have a say if Republicans let them.

And in the event a handful of Republicans break ranks, he might be able to find some votes among Democrats up in 2018 — several of whom are in states where he won, like West Virginia, North Dakota, Indiana and Montana — and want to show they are giving Trump a fair shake.

One, Joe Manchin of Trump-loving West Virginia, blasted Reid for calling Trump after the election a “sexual predator” among other things. He said he plans to support Trump’s executive nominations out of courtesy.

“I was a governor one time and I asked all my legislators, ‘judge me on how we perform, not on who I put there. They’re responsible to me,'” Manchin said. “I’m going to be receptive to whoever that team is.”

McConnell faces a different choice when it comes to Supreme Court picks. Nominations to the court still require 60 votes to advance in the Senate, meaning that Republicans would have to get support from at least eight Democrats — unless the GOP decides to extend the “nuclear option” to high court nominees.

Trump’s ability to place his choice on the court will be tested early because he’ll need to fill the vacancy created by the Feb. 13 death of Justice Antonin Scalia.

Laura Litvan contributed.

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