Sen. Mark Warner Mark Robert WarnerDemocrats call for declassifying election threats after briefing by Trump officials It's time to upgrade benefits Intelligence chief says Congress will get some in-person election security briefings MORE (D-Va.) issued a warning to President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE on Wednesday, telling the president to not interfere with special counsel Robert Mueller's "critical investigation" into Russian interference in the 2016 election.

Warner, the top Democrat on the Senate Intelligence Committee, in a speech on the Senate floor called for lawmakers to protect the probe.

"I believe it is up to every member of this institution, Republican or Democrat, to make a clear and unambiguous statement: that any attempt by this president to remove special counsel Mueller from his position, or to pardon key witnesses in an 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.

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"These are red lines, and we simply cannot allow them to be crossed," he added.

The Virginia senator argued that Mueller should be able to see the investigation entirely through without interference or obstruction from the administration. He said that if the administration attempts to influence the probe in any way, it would not only "call into question this administration's commitment to the truth," but also create a "constitutional crisis."

"In the United States of America, no one is above the law, not even the president. 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 stated.

His remarks come after questions swirled on Capitol Hill that Trump could be contemplating firing Mueller, as his investigation appears to pick up steam and reach into the president's tight inner circle.

Trump dismissed the murmurs, telling reporters on Sunday he is not planning to dismiss the special counsel.

Warner, however, cited the firing of former FBI Director James Comey in suggesting he did not trust Trump to keep his word.

"In recent days, the president has said he is not considering removing special counsel Mueller, but the president’s track record on this front is a source of concern. I’m certain many of my colleagues believed that he wouldn’t possibly fire Mr. Comey either," he said.

Warner also blasted Trump defenders who accuse Mueller of filling his investigative team with biased agents, pointing out how the head investigator, who is a “lifelong Republican,” was appointed by a Republican to lead the investigation.

“Indeed, there is much to praise. The fact is that Robert Mueller has impeccable credentials as a man of the law. He has assembled a team that includes some of the nation’s best investigators, and he is leading this investigation with the professionalism that it deserves,” he continued.

Warner also argued that Mueller showed he was a "fair and independent investigator" when he removed veteran FBI official Peter Strzok from his team upon learning that he had sent text messages critical of the president.

Warner said the Republicans who have seized on Strzok as an example that Mueller is leading a biased investigation have conveniently ignored “the fact that, as soon as Mr. Mueller learned about those comments, he immediately removed the agent in question from the investigation.”

His remarks come after Mueller brought a series of charges against former Trump campaign aides.

Michael Flynn, Trump's former national security adviser, pleaded guilty to lying to FBI agents earlier this month while also agreeing in a plea deal to fully cooperate with the Justice Department's investigation.