Republican Sen. Lindsey Graham said Tuesday that if President Donald Trump were to fire the special counsel Robert Mueller, he could risk impeachment.

Graham distinguished between Mueller's investigation and other controversial events at the Justice Department and the FBI that the president has used as evidence that the Mueller team is partisan.

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Republican Sen. Lindsey Graham said Tuesday that if President Donald Trump moved to fire the special counsel Robert Mueller, it would "probably" be an impeachable offense and would precipitate a constitutional crisis.

Graham, who has vacillated between being one of Trump's fiercest Republican critics and one of his most loyal defenders, told the conservative talk-radio host Hugh Hewitt that if the president fired Mueller without cause, it would most likely suggest ulterior motives.

"Well, I think what the president will have done is stopped an investigation in whether or not his campaign colluded with the Russians, what effect the Russians had on the 2016 campaign," Graham told Hewitt. "I can't see it being anything other than a corrupt purpose."

"I can't think of a more upsetting moment in the rule of law to have an investigator looking at a president's campaign as to whether or not they colluded with a foreign government, what kind of crimes may have been committed," Graham said. "I've seen no evidence of collusion, but to stop investigation without cause, I think, would be a constitutional crisis."

Graham seemed to echo other lawmakers like Democratic Sen. Dick Durbin, who used similar language over the weekend.

"If the president reaches out and stops this investigation, that is a constitutional crisis in this country," Durbin told Fox News' Chris Wallace on Sunday. "That's been said by Democrats and Republicans alike."

Mueller investigation separate from other grievances, Graham says

Graham went on to defend Trump's criticism of the conduct at the Department of Justice and the firing of Andrew McCabe, the deputy FBI director, over the weekend. But he separated those events from the Mueller investigation.

Andrew McCabe, the former FBI's former deputy director. Thomson Reuters

McCabe was forced to step down from his FBI post earlier this year amid an investigation by the Office of the Inspector General into his approval of disclosures to the media in October 2016 related to the bureau's investigation of the 2016 Democratic presidential nominee, Hillary Clinton.

The inspector general of the Justice Department, Michael Horowitz, reportedly said McCabe was not forthcoming during the review. The FBI Office of Professional Responsibility subsequently recommended that Attorney General Jeff Sessions terminate McCabe, according to The New York Times.

Trump's concerns about the Justice Department have largely echoed claims Rep. Devin Nunes made in a controversial memo released last month. The memo accused the Justice Department and FBI of improperly using the Russia dossier to obtain a surveillance warrant against a Trump campaign associate. Democrats have called the memo misleading and said the dossier was but one piece of the evidence presented.

But Graham said the events around the DOJ surveillance warrant took place well before Mueller was appointed special counsel in May.

"They're disconnected in time," Graham said on Sunday. "Mueller came along long after this. He's looking at things unrelated to the dossier. And we're a rule-of-law nation."

Trump, who has long opposed the Russia investigation, attacked Mueller by name over the weekend, tweeting on Saturday that the "Mueller probe should never have been started." That led to speculation he was moving to fire the special counsel.