WASHINGTON (Reuters) - The top Democrat on the U.S. Senate Intelligence Committee, responding to escalating Republican attacks on Special Counsel Robert Mueller, said on Wednesday that if President Donald Trump fires Mueller, it “has the potential to provoke a constitutional crisis.”

Speaking on the Senate floor, Senator Mark Warner denounced attacks on Mueller’s impartiality and said the special counsel’s investigation of ties between Trump’s presidential campaign and Russia must be “able to go on unimpeded.”

Russia denies meddling in the 2016 U.S. election and Trump has denied any collusion.

While Trump’s political allies have increased their criticism of Mueller, the president said on Sunday he was not considering firing him.

Republican lawmakers have seized on anti-Trump texts by a Federal Bureau of Investigation agent who was involved in the Russia investigation as evidence of bias in Mueller’s team. Mueller removed the agent from his team after the texts came to light.

Republicans on several House of Representatives committees have also announced their own probes into long-standing political grievances, including the FBI’s handling of Hillary Clinton’s use of a private email server when she was secretary of state. Clinton, a Democrat, was Trump’s opponent in last year’s election.

“Over the last several weeks, a growing chorus of irresponsible voices have called for President Trump to shut down Special Counsel Mueller’s investigation,” said Warner, adding that the attacks were “seemingly coordinated.”

“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 of rule of law,” Warner said. “It also has the potential to provoke a constitutional crisis.”

Warner called for Congress to make clear to the president that firing Mueller would have “immediate and significant consequences.”

House Democrats had circulated rumors last week that Trump would fire Mueller this Friday, just before the Christmas holiday.

Trump’s White House lawyer, Ty Cobb, said in a statement on Wednesday that the administration “willingly affirms yet again, as it has every day this week, there is no consideration being given to the termination of the special counsel.”

“If the media is going to continue to ask for responses to every absurd and baseless rumor, attention-seeking partisans will continue to spread them,” Cobb added.