Sen. Mark Warner Mark Robert WarnerIntelligence chief says Congress will get some in-person election security briefings Overnight Defense: Trump hosts Israel, UAE, Bahrain for historic signing l Air Force reveals it secretly built and flew new fighter jet l Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' House approves bill to secure internet-connected federal devices against cyber threats MORE (D-Va.) on Thursday warned that firing special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE is a line that President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE "cannot cross" following new reports that Trump ordered the White House counsel to fire Mueller last year.

“I’ve said it before, and I am saying it again: firing the Special Counsel is a red line that the President cannot cross," Warner said in a statement reacting to a bombshell report by The New York Times that Trump attempted to fire Mueller last summer.

Trump, who has repeatedly blasted the special counsel's probe into ties between the Trump campaign and Russia as a "witch hunt," reportedly backed down last June after White House counsel Don McGahn threatened to resign.

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"Any attempt to remove the Special Counsel, pardon key witnesses, or otherwise interfere in the investigation, would be a gross abuse of power, and all members of Congress, from both parties, have a responsibility to our Constitution and to our country to make that clear immediately," Warner said Thursday.

Warner is the top Democrat on the Senate Intelligence Committee, which is conducting its own investigation into Russian meddling. The Virginia senator has repeatedly criticized Trump for being "unwilling" to recognize the security threat posed by Russia.

Mueller reportedly learned of Trump's attempt to fire him, a move long cautioned against by both Republicans and Democrats in Congress, and is now investigating top officials from Trump's administration who are currently in office or who left over the past year.