(CNN) Republican Sen. Jeff Flake, an outspoken critic of President Donald Trump, tweeted Tuesday that if the President were to fire special counsel Robert Mueller, the "only constitutional remedy is after the fact, through impeachment."

"We are begging the president not to fire the special counsel," tweeted the Arizona Republican, who is not running for reelection. "Don't create a constitutional crisis. Congress cannot preempt such a firing. Our only constitutional remedy is after the fact, through impeachment. No one wants that outcome. Mr. President, please don't go there."

Concerns about the President potentially firing the special counsel grew over the weekend after Trump criticized Mueller by name on Twitter and his lawyer said he prayed for an end to the investigation into Russian meddling and possible collusion between the Trump campaign and Russian officials.

Flake, who sits on the Senate Judiciary Committee, has been hesitant this week whether to back existing legislation that would protect the special counsel from political pressure at the White House, saying there were some constitutional questions about the legislation.

His tweet Tuesday indicated that he doesn't believe Congress could legislate a law that would protect the special counsel.

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