The Senate Intelligence Committee’s top Democrat said Thursday that President Donald Trump would spark a “constitutional crisis” if he fired special counsel Robert Mueller or Deputy Attorney General Rod Rosenstein.

“I believe that firing Mueller or Rosenstein would create a constitutional crisis,” Sen. Mark Warner, D-Va., told the PBS NewsHour’s Judy Woodruff. Warner added that “history would then judge all of us” if Congress did not prevent Trump from firing the officials in charge of overseeing the FBI’s investigation into Russian meddling in the 2016 election and possible ties to the Trump campaign.

The White House has in the past denied that Trump is considering firing Mueller. Rosenstein appointed the former FBI director last May to head the Russia probe.

But Warner said the president is unpredictable. Last week, as FBI agents raided Michael Cohen, the president’s personal lawyer, Trump said “we’ll see what happens” when asked if he’d fire Mueller. And White House press secretary Sarah Huckabee Sanders said the president had the power to fire Mueller, if he wanted to.

“Saying one thing on one day doesn’t mean that’ll be his position the next day,” Warner said of Trump.

Warner, who along with Sen. Richard Burr, R-N.C, is leading the Senate Intelligence panel’s probe into Russia — also expressed support for a bipartisan Senate bill aimed at protecting Mueller from getting fired. The bill would fast track a judicial review of a president’s decision to fire Mueller and future special counsels.

Senate Judiciary Chairman Chuck Grassley, R-Iowa, is planning to hold a committee vote on the bill. But Senate Majority Leader Mitch McConnell, R-Ky., has said he won’t bring the bill to the full Senate floor, so its chances of passage, at least now, appear to be zero.

Warner said he was disappointed in McConnell’s opposition to the proposal. Warner added that he hoped the majority leader “would reconsider” if the Judiciary Committee passes the measure with overwhelming bipartisan support.