(CNN) Sen. Lindsey Graham gave a stern warning Sunday to President Donald Trump against firing special counsel Robert Mueller.

"As I said before, if he tried to do that, that would be the beginning of the end of his presidency," the South Carolina Republican said on CNN's "State of the Union."

"The only reason that Mr. Mueller could be dismissed is for cause. I see no cause when it comes to Mr. Mueller," Graham said, later adding he believed the Mueller was "doing a good job."

Graham called for Mueller to be able to carry out his investigation into Russian interference in the 2016 election "independent of any political influence."

"I pledge to the American people, as a Republican, to make sure that Mr. Mueller can continue to do his job without any interference," he said.