Sen. Lindsey Graham, R-S.C., said Sunday it would be the end of President Trump's time in office if he fired special counsel Robert Mueller, and everyone in his White House knows it.

Graham said he understood Trump's frustration with the ongoing Russia probe, adding most presidents want "to get rid of critics."

"I'm sure that there'll be an investigation around whether or not President Trump did try to fire Mr. Mueller. We know that he didn't fire Mr. Mueller. We know that if he tried to, it would be the end of his presidency," Graham said. "So at the end of the day, let Mr. Mueller do his job."

Graham and Trump have grown closer as friends during Trump's first year in office after they spent much of the 2016 election cycle bashing each other. However, Graham has found himself on the outs with the White House recently over immigration negotiations.

Graham was a member of the House when it voted to impeach President Bill Clinton.

Graham said he isn't going to dismiss reports that President Trump wanted to fire special counsel Robert Mueller as "fake news," urging Mueller to investigate the claims himself.

"I believe it's something that Mueller should look at," Graham told ABC's "This Week."

“We’re not just going to say it’s 'fake news' and move on.”

"I have complete confidence in Mr. Mueller," Graham continued. "I will do whatever it takes to make sure that Mr. Mueller can do his job. We were a rule of law nation before President Trump, we're going to be a rule of law nation after President Trump."

Graham also said he would persist with efforts to pass legislation in the Senate to protect Mueller from unfair or unwarranted dismissals.