John Bolton, President-elect Trump’s former national security adviser, is warning that Republican senators will be judged harshly by history if they confirm Kash Patel as the next FBI director.

Patel, who has called for shuttering the FBI’s headquarters in Washington and firing its top ranks, is picking up a lot of early Senate Republican support and appears to be cruising toward confirmation.

But Bolton warns that GOP senators would make a huge mistake if they put Patel, a staunch Trump loyalist, atop the nation’s premier law enforcement agency.

Bolton, who was familiar with Patel’s work at the National Security Council (NSC) during Trump’s first term, says he didn’t fully realize years ago how much of a “threat” Patel posed to the Constitution and country because of his willingness to place loyalty to Trump above all else.

But he says now there’s now plenty of evidence to know how Patel would handle one of the nation’s top law enforcement jobs. And he urged GOP senators to pay heed.

“With more facts available and less rhetoric, the result will be clear. I regret I didn’t fully discern Mr. Patel’s threat immediately. But we are now all fairly warned. Senators won’t escape history’s judgment if they vote to confirm him,” Bolton wrote in a Wall Street Journal op-ed published Wednesday.

Bolton says Patel “proved to be less interested in his assigned duties than in worming his way into Mr. Trump’s presence” during his stint at the NSC during Trump’s first term.

“His conduct in Mr. Trump’s first term and thereafter indicates that as FBI director he would operate according to Lavrenty Beria’s reported comment to Joseph Stalin: ‘Show me the man, and I’ll show you the crime,’” he wrote, referring to the Soviet Union’s infamous minister of internal affairs.

He said Patel was brought on at the NSC during Trump’s first term despite a “lack of policy credentials” because “the president ordered him hired.”

Bolton says Patel was never in charge of a directorate during his time at the NSC despite his lofty claim in a memoir that he was in charge.

“He reported to senior directors … and had defined responsibilities. His puffery was characteristic of the résumé inflation we detected when Mr. Trump pressed him on us,” Bolton wrote. “Given the sensitivity of the NSC’s responsibilities, problems of credibility or reliability would ordinarily disqualify any job applicant.”

Bolton noted former Defense Secretary Mark Esper’s claim in a memoir that Patel put a hostage-rescue operation in 2020 at risk by falsely assuring the State Department that the Nigerian government had been informed of the operation.

“Mr. Esper writes, the clearance hadn’t been obtained, threatening the operation’s success, and his team ‘suspected Patel made the approval story up’ but wasn’t certain,” he wrote, citing the former Defense secretary’s account.

Bolton highlighted that Olivia Troye, who served as a counterterrorism adviser to former Vice President Mike Pence, recently “elaborated on these concerns” by “tagging Mr. Patel with ‘making things up on operations’ and lying about intelligence.”

“These are but a few of many cases that touch directly on Mr. Patel’s character and his consistent approach of placing obedience to Mr. Trump above other, higher considerations—most important, loyalty to the Constitution,” he wrote.