Rep. Jackie Speier Karen (Jackie) Lorraine Jacqueline SpeierOvernight Defense: House to vote on military justice bill spurred by Vanessa Guillén death | Biden courts veterans after Trump's military controversies House to vote on 'I Am Vanessa Guillén' bill Overnight Defense: Trump's battle with Pentagon poses risks in November | Lawmakers launch Fort Hood probe | Military members can't opt out of tax deferral MORE (D-Calif.) on Sunday warned that if 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 orders the firing of 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, she will vote to impeach him.

“The President is careening us toward what I fear will be a constitutional crisis. Mr. President: Here is my red line—Fire Mueller and I will vote to fire you,” Speier tweeted.

The President is careening us toward what I fear will be a constitutional crisis. Mr. President: Here is my red line—Fire Mueller and I will vote to fire you. — Jackie Speier (@RepSpeier) March 18, 2018

Speier’s comment comes amid growing speculation that Trump is moving toward firing Mueller, who is investigating Russian interference in the 2016 election.

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Trump has at various times this weekend lashed out at the FBI, Department of Justice and Mueller probe.

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 Lindsey Olin GrahamMcConnell locks down key GOP votes in Supreme Court fight Will Republicans' rank hypocrisy hinder their rush to replace Ginsburg? Momentum growing among Republicans for Supreme Court vote before Election Day MORE (R-S.C.) suggested such an action would be "the beginning of the end" of Trump's presidency.