“We are begging the President not to fire the special counsel. Don't create a constitutional crisis. Congress cannot preempt such a firing. Our only constitutional remedy is after the fact, through impeachment,” Arizona Republican Jeff Flake said on Twitter.
“No one wants that outcome. Mr. President, please don't go there."
Already a frequent target for Mr Trump’s scorn, Mr Flake has wielded limited clout since announcing he would retire from Congress after his term expires.
But while other Republicans have implored Mr Trump to not fire Mr Mueller - South Carolina Republican Lindsey Graham said last week it would be “the beginning of the end of his presidency” - Mr Flake’s statement was extraordinary for embracing the possibility of impeachment.
By echoing the language of Democrats who have endorsed removing Mr Trump from office, Mr Flake illuminated the stakes if Mr Trump does choose to dismiss the man tasked with investigating members of the President’s circle.
For months, Democrats have warned that Mr Trump would risk a constitutional crisis if he seeks to jettison Mr Mueller, who is leading an investigation into potential coordination between the Russian government and Mr Trump’s presidential bid.
Mr Mueller was tabbed to lead the investigation after Mr Trump fired then-FBI director James Comey and went on to connect the dismissal to “this Russia thing”.
The President has regularly assailed Mr Mueller’s investigation, saying it has been irremediably tainted by bipartisan bias. He has emphatically rejected the suggestion of collusion.
The probe has produced indictments of multiple former campaign aides to Mr Trump and charges against a collection of Russian nationals accused of orchestrating an online influence campaign with the aim of boosting Mr Trump.