Sen. Mark Warner, D-Va., said on Wednesday that he is concerned about a "growing chorus of irresponsible and reckless" voices that are calling for an end to special counsel Robert Mueller's investigation of the president's ties to Russia.
Warner, the vice chairman of the Senate Intelligence Committee, said in a speech on the Senate floor that "in recent weeks those voices seem to be growing in stridency and volume."
Calling Mueller a "fair and independent investigator," Warner said he was worried about the "seemingly coordinated" nature of the critiques on the investigation.
The former Virginia governor has been vocal in his support of the special counsel's office since Mueller was appointed in May.
In an interview published Tuesday, Warner told CNBC that firing Mueller could lead to a "constitutional crisis."
"If they pull on one of these threads as a reason to fire Mueller, I think it will be a political disaster for the president, and I believe it will be a constitutional crisis," he said.
President Donald Trump has said repeatedly there was no misconduct in his campaign's dealings with Russia and that he would comply with Mueller's investigation. On Sunday, he repeated that he had no plans to fire Mueller and stressed that there was "no collusion."
Nonetheless, tensions between the special counsel and the president have escalated in recent weeks.
In a Saturday letter, Trump transition attorney Kory Langhofer accused the special counsel's office of unlawfully obtaining thousands of emails. The special counsel's office said that its actions were legal.
Senior Republicans had already been criticizing the investigation over perceived bias among investigators. Following the revelation of anti-Trump text messages sent by former investigators working on the probe, conservative lawmakers grilled Deputy Attorney General Rod Rosenstein in an oversight hearing.
The texts, exchanged by Peter Strzok and Lisa Page, show the two senior FBI officials calling the president an "idiot," among other disparaging comments.
"Reports on the political predisposition, and potential bias, of certain career agents and department lawyers on special counsel Mueller's team are deeply troubling to all citizens who expect a system of blind and equal justice," House Judiciary Chairman Bob Goodlatte, R-Va., said at the hearing.
Separately, Republicans on the Senate Intelligence Committee are reportedly nearly ready to end their investigation despite pleas from Democrats to keep pushing forward.
That investigation was underway when Mueller was appointed in May. At the time, Warner praised the former FBI director's appointment.