Sen. Mark Warner, the top Democrat on the Senate Intelligence Committee, said Wednesday that the firing of Justice Department special counsel Robert Mueller would be a "gross abuse of power" and called for Congress to respond with "significant consequences" if President Donald Trump took that step.
Warner, a Virginia Democrat, took to the Senate floor for a speech, in which, he said the firing of Mueller or the pardoning of those charged in the special counsel's investigation would represent crossing his "red line."
"Any attempt by this President to remove special counsel Mueller from his position or to pardon key witnesses in any effort to shield them from accountability or shut down the investigation would be a gross abuse of power and a flagrant violation of executive branch responsibilities and authorities," Warner said. "These truly are red lines and simply cannot allow them to be crossed."
"Congress must make clear to the President that firing the special counsel or interfering with his investigation by issuing pardons of essential witnesses is unacceptable and would have immediate and significant consequences," he added.
Warner's speech comes as Mueller is facing a growing number of Republicans who have been critical Mueller's probe for being biased against Trump. They've pointed to the text messages of FBI agent Peter Strzok, who was removed from Mueller's team after the anti-Trump messages surfaced.
Warner argued that the congressional probes into possible collusion between the Trump team and Russian officials -- the Senate Intelligence Committee, House Intelligence Committee and Senate Judiciary Committee all have open investigations -- were not the same as Mueller's investigation because Congress was not investigating potential criminal wrongdoing.
"It should be very clear that our committee cannot and will not stand as a substitute for Mr. Mueller's investigation," Warner said.
CNN has reached out to the White House for comment on Warner's warning but has not yet received a response.
On Sunday, Trump told reporters he's not considering firing Mueller, but added that his "people" are "very upset" about the counsel's decision to obtain tens of thousands of emails from the Trump transition team.
After his speech, Warner told reporters he was motivated to deliver his speech before lawmakers head home for Christmas because he sees a "coordinate effort to undermine Mueller, and in many ways more broadly even the FBI."
But Warner would not discuss specifics about what he thought should be done if his red line was crossed, and Mueller was fired or those charged in the probe were pardoned.
When asked if crossing his red line was an impeachable offense, Warner said: "My hope is that other members will echo the call I made on both sides of the aisle, and I hope that the President will adhere to what he's said recently that he has no intentions."
Democrats have been pushing for the Senate to take action on legislation to protect the special counsel from being fired by the President. There are currently two bipartisan bills introduced in the Senate to do so, and the senators involved are negotiating to combine them into one measure.