If Donald Trump moves to fire special counsel Robert Mueller over his federal investigation into potential ties between Trump campaign officials and Russia he would provoke a “constitutional crisis,” a leading Democrat has warned.

“If you were to see a firing, I think you would see a constitutional crisis” that would also be “a political disaster for the President,” Senator Mark Warner of Virginia said in an interview with CNBC. Mr Warner sits on the Senate Intelligence Committee, which is also looking into Russian meddling in the 2016 presidential election and possible ties to Mr Trump’s campaign.

Critics of the President have invoked the risk of a constitutional crisis as Mr Mueller’s investigation tightens around Mr Trump’s inner orbit, warning that a move to fire Mr Mueller would undercut the principle that even presidents are not immune from legal repercussions.

The president said over the weekend he had no intention of firing Mr Mueller. But he has regularly railed against an investigation that he calls a partisan “witch hunt”, rejecting any allegations of possible collusion with Russia.

Appointed to lead the federal investigation into Russia after Mr Trump dismissed former FBI director James Comey, Mr Mueller has increasingly become a target for conservative ire.

Some Republicans have called for Mr Mueller to step aside after the Department of Justice released text messages in which an FBI agent who would later work on Mr Mueller’s investigation disparaged Mr Trump.

The agent was fired after the communications came to light. While deputy Attorney General Rod Rosenstein has defended the investigation’s integrity during recent testimony before Congress following pointed questions from Republicans about FBI agents’ history of donations to political causes they said aligned with Democrats.

“We recognize we have employees with political opinions. It’s our responsibility to make sure those opinions do not influence their actions,’’ Mr Rosenstein said. “I believe that Director Mueller understands that, and he is running his office appropriately.”

Before dismissing Mr Comey earlier this year, Mr Trump allegedly asked the then-FBI chief to “let go” the investigation into the president’s former National Security Adviser Michael Flynn. Mr Comey raised the conversation during testimony to Congress, but Mr Trump and the White House have repeatedly denied it took place. 

Mr Trump later cited “this Russia thing” as a reason for Mr Comey’s dismissal. Mr Flynn is one of a number former Trump campaign aides to face charges as part of Mr Mueller’s investigation. Mr Flynn pleaded guilty to one count of lying to the FBI. 

Noting that precedent, Democrats have worried that Mr Mueller could be next. During his interview, Mr Warner decried “cheap shots when some of these Republican colleagues would question Mueller’s integrity”. 

For his part, Mr Rosenstein said he would have to evaluate an order from Mr Trump to jettison Mr Mueller.

“If there were good cause, I would act,” Mr Rosenstein told Congress. “If there were no good cause, I would not”.