WASHINGTON (CN) - Hours after President Donald Trump ousted Attorney General Pam Bondi, questions swirl about who the White House will tap to become the government's new top law enforcement official.

But no matter who the president nominates to fill that role, they'll have to survive a contentious confirmation process in the Senate. And the success of that nominee could hinge on just one man: North Carolina Senator Thom Tillis.

Tillis, who announced last year he would retire at the end of his term, has vowed to call balls and strikes on the Trump administration with his remaining time on Capitol Hill. As a member of the Senate Judiciary Committee, he's demonstrated a willingness to break with the White House and block its nominees for top law enforcement roles, often to the chagrin of the president himself.

The North Carolina senator has long held up the Jan. 6, 2021, Capitol riot as a key red line in his consideration of Trump administration nominees. And in the wake of Bondi's firing Thursday, Tillis put the issue front and center as he discussed the attorney general's possible replacement, saying he would not support any nominee who justified "any element" of Jan. 6.

"For me, the threshold for somebody following Pam Bondi ends the moment I hear they said one thing that excused the events of Jan. 6," the senator told CNN Thursday night. "I've been very clear on that - so I hope whoever they have in mind to follow General Bondi is very clear-eyed on my position on Jan. 6."

Tillis' threat is not unsubstantiated. Last year he singlehandedly tanked Ed Martin, Trump's nominee to become U.S. attorney for D.C., over past statements about the prosecution of Capitol rioters. Martin, at the time the capital city's acting U.S. attorney, opened an investigation into his office's handling of Jan. 6 cases and had defended people charged with breaching the Capitol.

"None of that stuff resonates with me," Tillis said at the time, slamming Martin's fixation on what he called the "over-prosecution" of Jan. 6 rioters and arguing anyone who entered the Capitol during the riot should be "prosecuted, period, full stop, no rationalization."

Tillis later said any Trump nominee who condoned violence on Jan. 6 "need not have a hearing" in the Judiciary Committee, because they would not have his support. And he's been critical of others who have testified before the panel and refused to directly answer questions about the Capitol riot.

"We're talking about people who are all supposed to come in, be objective, follow the Constitution," he told Courthouse News in December after a trio of Trump nominees sidestepped questions about Jan. 6 during a Judiciary Committee hearing. "If you're acting like a politician, expect to be treated like one."

But the senator has also been willing to look past the comments of some administration nominees who have been more artful in their refusal to condemn the actions of Jan. 6 rioters or the effort to overturn the results of the 2020 presidential election.

Tillis voted to advance Third Circuit Judge Emil Bove out of the Judiciary Committee last summer despite the then-nominee's veiled criticism of the Justice Department's prosecution of Capitol rioters. The lawmaker said at the time he couldn't find "one piece of evidence" that Bove had condoned violence on Jan. 6, adding that Congress should distinguish between "thugs that I believe should still be in prison" and other "boneheads" who entered the Capitol during the riot.

Bove, now an appellate judge, previously referred to the prosecutions of Jan. 6 rioters as a "grave national injustice" and criticized what he called "overreach and heavy-handed tactics by prosecutors and law enforcement." The former Justice Department official and member of Trump's personal legal team also cast doubt on the characterization of the Capitol riot, saying it was "a matter of significant political debate."

Tillis told reporters at the time he didn't believe the Third Circuit nominee's comments rose to the level of excusing the behavior of Jan. 6 rioters.

A spokesperson for Tillis' office did not immediately return a request for comment on what sort of comments about the Capitol riot he'd consider disqualifying for an attorney general nominee.

Beyond his Jan. 6 litmus test for nominees, Tillis has also been willing to publicly break with the Trump administration on major issues. The North Carolina senator has said he will not vote for any of the president's nominees for vacancies on the Federal Reserve - including its chairman - until the Justice Department ends a perjury probe into Jerome Powell, the central bank's current top official.

Tillis was also one of the few Republicans to call for the resignations of former Homeland Security Secretary Kristi Noem and senior White House adviser Stephen Miller amid the administration's immigration crackdown in Minnesota that saw federal agents kill two American citizens. He excoriated Noem at a hearing in March, calling her leadership a "disaster."

Trump fired Noem shortly after that hearing. But the president has been furious with Tillis' meddling, calling him a "loser" in a January social media post for demanding the DHS leader and Miller step down.

"I'm thrilled about that," the senator said in response. "It makes me qualified to be homeland security secretary and senior adviser to the president."

Meanwhile, the Trump administration has yet to announce who will replace Bondi at the head of the Justice Department. Deputy Attorney General Todd Blanche will fill in for the outgoing attorney general in an acting capacity - though Bondi has signaled she will stay on through the month to assist with the transition.

Environmental Protection Agency administrator Lee Zeldin and U.S. Attorney for D.C. Jeanine Pirro have both been reported as potential attorney general nominees. And Alina Habba, Trump's former personal lawyer and disqualified pick to serve as U.S. attorney for the District of New Jersey, posted a phot on social media Friday of her meeting the president in the Oval Office.

The White House did not return a request for comment on whether Trump was considering nominating Habba as attorney general.

Source: Courthouse News Service