Sen. John McCain John Sidney McCainMomentum growing among Republicans for Supreme Court vote before Election Day McConnell urges GOP senators to 'keep your powder dry' on Supreme Court vacancy McSally says current Senate should vote on Trump nominee MORE (R-Ariz.) on Sunday praised special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE amid speculation that President Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE may be preparing to fire him.

“Special Counsel Mueller has served our country with honesty and integrity. It’s critical he be allowed to complete a thorough investigation into Russia’s interference in the 2016 election — unimpeded,” McCain tweeted.

Special Counsel Mueller has served our country with honesty and integrity. It’s critical he be allowed to complete a thorough investigation into Russia’s interference in the 2016 election — unimpeded. — John McCain (@SenJohnMcCain) March 18, 2018

Trump has at various times this weekend lashed out at the FBI, Department of Justice and Mueller probe, prompting speculation he might call for Mueller's firing.

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On Saturday, he asserted that Mueller's investigation should never have been opened in the first place. On Sunday, he attacked the makeup of Mueller's team, claiming it is unfairly stacked with Democrats. Mueller himself is a Republican.

The tweets came shortly after Trump's lawyer, John Dowd, called on Deputy Attorney General Rod Rosenstein Rod RosensteinDOJ kept investigators from completing probe of Trump ties to Russia: report Five takeaways from final Senate Intel Russia report FBI officials hid copies of Russia probe documents fearing Trump interference: book MORE, who oversees Mueller's investigation, to shut down the probe.

On Sunday, lawmakers from both sides of the aisle warned Trump against firing the special counsel. Sen. Lindsey Graham (R-S.C.) suggested such an action would be "the beginning of the end" of Trump's presidency.

Mueller has obtained guilty pleas from three former Trump associates, and indicted former Trump campaign chairman Paul Manafort Paul John ManafortOur Constitution is under attack by Attorney General William Barr Bannon trial date set in alleged border wall scam Conspicuous by their absence from the Republican Convention MORE. He also filed charges against 13 Russians and three Russian organizations for allegedly interfering in the 2016 election.