Sen. Dianne Feinstein Dianne Emiel FeinsteinTrump faces tricky choice on Supreme Court pick The Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' Abortion stirs GOP tensions in Supreme Court fight MORE (D-Calif.) said Friday she is "growing increasingly concerned" that President Trump will try to fire special counsel Robert Mueller, who is overseeing the investigation into Russian meddling in the presidential election, and Deputy Attorney General Rod Rosenstein.

“The message the president is sending through his tweets is that he believes the rule of law doesn’t apply to him and that anyone who thinks otherwise will be fired. That’s undemocratic on its face and a blatant violation of the president’s oath of office," Feinstein, who the top Democrat on the Judiciary Committee, said in a statement on Friday.

Trump has frequently blasted the Russia investigation, which includes potential ties between the Trump campaign and Moscow, on social media. He tweeted on Friday morning that he is "being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt."

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Pointing to Trump's latest tweets, Feinstein argued that Trump has "embarked on an effort to undermine anyone with the ability to bring any misdeeds to light."

"The Senate should not let that happen. We’re a nation of laws that apply equally to everyone, a lesson the president would be wise to learn," Feinstein said.

Trump can't directly fire Mueller, but could direct Rosenstein — who appointed him as special counsel — to fire him. Rosenstein testified this week that he had hadn't seen "good cause" for firing Mueller and would only do so "I am not going to follow any orders unless I believe those are lawful and appropriate,” Rosenstein said, explaining that under federal regulations, Mueller can only be fired for good cause and that reason would have to be put in writing.

If Rosenstein refused, Trump could fire him and try to appoint someone, who would require Senate confirmation, that would agree to fire Mueller.

But Feinstein warned against that strategy predicting Senate Republicans, who publicly warned Trump this week against firing Mueller, would revolt.

"If the president thinks he can fire Deputy Attorney General Rosenstein and replace him with someone who will shut down the investigation, he’s in for a rude awakening. Even his staunchest supporters will balk at such a blatant effort to subvert the law," she said.

Rosenstein has privately told colleagues that he could have to recuse himself from the special investigation, according to ABC News.

The Washington Post reported that Mueller is investigating if Trump tried to obstruct the FBI's Russia investigation.