Graham warns Trump: Firing Mueller would be 'beginning of the end of his presidency'

Sen. Lindsey GrahamLindsey Olin GrahamRestlessness, light rule-breaking and milk spotted on Senate floor as impeachment trial rolls on Senator-jurors who may not be impartial? Remove them for cause Broad, bipartisan rebuke for proposal to pull troops from Africa MORE (R-S.C.) on Sunday warned of the consequences for President TrumpDonald John TrumpMnuchin knocks Greta Thunberg's activism: Study economics and then 'come back' to us The Hill's Morning Report - House prosecutes Trump as 'lawless,' 'corrupt' What to watch for on Day 3 of Senate impeachment trial MORE if he tries to fire special counsel Robert MuellerRobert (Bob) Swan MuellerSchiff: Trump acquittal in Senate trial would not signal a 'failure' Jeffries blasts Trump for attack on Thunberg at impeachment hearing Live coverage: House Judiciary to vote on impeachment after surprise delay MORE.

Graham was asked on CNN's "State of the Union" if he is worried Trump is preparing to order Mueller's firing.

"If he tried to do that, that would be the beginning of the end of his presidency, because we're a rule of law nation," Graham said.

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Graham said Mueller is following the evidence where it takes him.

"I think it's very important he be allowed to do his job without interference and there are many Republicans who share my view," Graham said.

Trump has repeatedly dismissed the special counsel's probe into Russia's election interference and possible ties between his campaign and Russia as a "witch hunt" and has denied collusion.

Graham's comments come after Attorney General Jeff SessionsJefferson (Jeff) Beauregard SessionsLawmaker wants Chinese news outlet to register as foreign agent Trump-aligned group launches ad campaign hitting Doug Jones on impeachment ICE subpoenas Denver law enforcement: report MORE on Friday fired former FBI Deputy Director Andrew McCabeAndrew George McCabeMcCabe accuses Trump officials of withholding evidence in lawsuit over firing McCabe: Being accused of treason by Trump 'quite honestly terrifying' Horowitz report is damning for the FBI and unsettling for the rest of us MORE.

In recent days, Trump has taken to Twitter to attack the Mueller probe.

On Saturday, he tweeted the probe "should never have been started in that there was no collusion and there was no crime."

"It was based on fraudulent activities and a Fake Dossier paid for by Crooked Hillary and the [Democratic National Committee], and improperly used in [Foreign Intelligence Surveillance Act] COURT for surveillance of my campaign. WITCH HUNT!" Trump tweeted.

He also called McCabe's firing a "great day for the hard working men and women of the FBI."