President Donald Trump’s plan to promote acting U.S. Attorney for the District of Columbia Ed Martin, who has repeatedly defended Jan. 6 rioters, to the permanent position appears to face an uphill battle after a key Senate Republican said Tuesday he would not support the nomination.

Sen. Thom Tillis of North Carolina told ABC News that he will not support Trump’s nominee to become the next U.S. attorney for Washington, D.C., a message he says has been relayed to the White House.

“At this point, I have indicated to the White House I wouldn’t support his nomination,” Tillis told ABC News.

Tillis met with Martin Monday evening after he publicly expressed reluctance about Martin’s nomination due to concerns about his work representing Jan. 6 defendants and past inflammatory comments about the riot.

Martin, Tillis said, did a “good job of explaining how there were people that probably got caught up in it,” but that it wasn’t enough to satisfy his concerns.

“I think anybody who breached the perimeter should have been in prison for some period of time,” Tillis said. “Whether it’s 30 days or three years is debatable, but I have no tolerance for anybody who entered the building on January the sixth, and that’s probably where most of the friction was,” Tillis said.

Tillis is a key Republican vote on the Senate Judiciary Committee that will be responsible for determining whether to advance Martin’s nomination to a final vote on the Senate floor.

Martin has served as the interim U.S. attorney since the start of the administration but his term expires on May 20, meaning he would need to be confirmed by then to continue leading the office.

One path forward is for the chief judge of the D.C. District Court, Judge Jeb Boasberg, to pick the next nominee.

That is extremely unlikely given Trump’s apparent disdain for Boasberg who has overseen a number of Trump’s related cases.

Boasberg recently found probable cause that the Trump administration acted in contempt of court when officials in March ignored his order to turn around two planes carrying alleged Venezuelan gang members to El Salvador.

If Tillis votes with all Democrats to oppose the nomination and no other Republicans flip, the committee will tie, and Martin’s nomination won’t be sent to the floor for a final vote.

But there’s still a few ways to get around this. Tillis could choose to vote to advance without recommendation, allowing the nomination to go to the floor of the Senate even without Tillis’ support.

Losing any more Republicans on the panel could prove insurmountable.

The timing of a Judiciary Committee vote is also in flux at the moment, as a source close to the confirmation process confirms that Judiciary Chairman Chuck Grassley has not listed a committee vote for Martin on the committee’s agenda this week amid growing concerns about his floundering support.

This delay could make it close to impossible for the Senate to take a final vote on Martin before May 20.

Martin, who has been vying to become the top prosecutor in one of the nation’s most important U.S. attorney’s offices, was a promoter of Donald Trump’s “Stop the Steal” campaign and was himself seen on the Capitol grounds during the attack on the U.S. Capitol.

His tenure leading the office has been fraught with controversy.

Martin has moved to fire or demote dozens of top career attorneys who investigated the Capitol attack, he has sent threatening letters to top Democrats and other political opponents of President Trump suggesting he would target them with criminal investigations.

He recently had to apologize for his past praise of a Jan. 6 rioter who had a lengthy history of antisemitic statements and who infamously posted photos of himself dressed as Adolf Hitler.

Martin has also had to provide multiple supplemental letters to the committee in recent weeks after failing to disclose numerous media appearances on far-right outlets like InfoWars and Russian-propaganda networks Sputnik and RT.

Trump urged Republicans to vote for Martin, saying on his social media platform Monday evening that “if approved, HE WILL NOT LET YOU DOWN.”