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(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.

Graham’s defense of Mueller comes in the wake of the President’s latest attacks suggesting partisan bias on the special counsel.

“Why does the Mueller team have 13 hardened Democrats, some big Crooked Hillary supporters, and Zero Republicans? Another Dem recently added … does anyone think this is fair? And yet, there is NO COLLUSION!,” Trump wrote on Twitter.

Trump’s criticism of Mueller, a Republican, also came a day after his personal lawyer, John Dowd, called for an end to the special counsel’s probe.

On Saturday, CNN learned that Mueller had interviewed former FBI Deputy Director Andrew McCabe and asked about the firing of FBI Director James Comey. McCabe was fired by Attorney General Jeff Sessions on Friday night after being long derided by the President.

Graham said on “State of the Union” that the Senate Judiciary Committee should hold a hearing on McCabe’s firing to “make sure it was not politically motivated.”

“I think we owe it to the average American to have a hearing in the Judiciary Committee, for Mr. Sessions, Attorney General Sessions, comes forward with whatever documentation he has about the firing and give Mr. McCabe a chance to defend himself,” Graham said. “I believe when it comes to this issue, we need as much transparency as possible, to make sure that it was not politically motivated.

Democratic Sen. Patrick Leahy of Vermont issued a similar call to Senate Judiciary Committee Chairman Chuck Grassley on Saturday.