The top Democrat on the Senate Intelligence Committee warned Tuesday there would be a constitutional crisis if President Trump fires special counsel Robert Mueller.

“If this president allows this investigation to come to its conclusion and either bring charges or not, then I think the system will have worked as our founders intended,” Sen. Mark Warner, D-Va., told CNBC. “If they pull on one of these threads as a reason to fire Mueller, I think it will be a political disaster for the president, and I believe it will be a constitutional crisis.”

Trump has long dismissed Mueller’s probe into Russian meddling in the 2016 election as a “witch hunt." But he has recently stepped up his criticisms of the FBI after it was revealed a longtime FBI investigator working on the bureau’s probe into Hillary Clinton’s use of a private email server exchanged texts with an FBI lawyer mocking Trump.

The investigator, Peter Strzok, was assigned to work on Mueller’s probe into Russian meddling, but the special counsel removed him when he learned of the messages.

“It’s a shame what’s happened with the FBI,” Trump told reporters Friday, though he didn’t elaborate on specifics. “It’s a very sad thing to watch.”

Republican lawmakers have also ramped up their criticisms of Mueller’s investigation after the text messages between Strzok and lawyer Lisa Page were released.

“How, with a straight face, can you say this group of Democrat partisans are unbiased and will give President Trump a fair shake?” Rep. Steve Chabot, R-Ohio, asked Deputy Attorney General Rod Rosenstein regarding Mueller’s team during a House Judiciary Committee hearing last week.

Warner said he has been disappointed to see the shift in GOP lawmakers, who praised Mueller when he was tapped to lead the Russia investigation in May.

“When Bob Mueller was appointed, he was greeted with universal acclaim. I think it’s not fair. I think it’s not right. I think it’s frankly cheap shots when some of these Republican colleagues would question Mueller’s integrity,” he said. “And if you were to see a firing, I think you would see a constitutional crisis.”

Trump told reporters Friday he does not intend to fire Mueller.