The top Democrat on the Senate Intelligence Committee, Mark Warner of Virginia, took to the Senate floor Wednesday to warn the Trump administration against firing special counsel Robert Mueller, who is leading the investigation into whether the Trump campaign colluded with Russia during the 2016 presidential election.

Sen. Mark Warner: “Firing Mr. Mueller or any other of the top brass involved in this investigation would not only call into question this administration’s commitment to the truth, but also to our most basic concept, rule of law. It also has the potential to provoke a constitutional crisis. In the United States of America, no one—no one—is above the law, not even the president.”

Senator Warner’s comments come as a number of Republicans are criticizing the Mueller investigation. Warner says he’s concerned the Trump administration might fire Mueller during Congress’s Christmas recess. The White House said Wednesday it is not planning to fire Mueller.