The top Democrat on the Senate Judiciary Committee said she's worried President Trump will try to fire Robert Mueller and 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 it's face and a blatant violation of the president's oath of office," Sen. Dianne Feinstein, D-Calif., said in a Friday statement.

In an early morning tweet on Friday, Trump appeared to confirm reports that he's under investigation for obstruction of justice. Trump also attacked Deputy Attorney General Rosenstein, who appointed Mueller to the position.

Rosenstein wrote a memo outlining an argument against former FBI Director James Comey, which the White House later claimed contributed to the decision by Trump to fire Comey. However, Trump has said in an interview that he would have fired Comey "regardless" of Rosenstein's memo.

Feinstein said if Trump thinks he can fire Rosenstein "and replace him with someone who will shut down the investigation, he's in for a rude awakening."

"It's becoming clear to me that the president has embarked on an effort to undermine anyone with the ability to bring any misdeeds to light. We're a nation of laws that apply equally to everyone, a lesson the president would be wise to learn," Feinstein said.

Rosenstein said this week that he has not seen "good cause" to fire Mueller.

"I am not going to follow any orders unless I believe those are lawful and appropriate," Rosenstein told Sen. Susan Collins, R-Maine, during testimony before a Senate Appropriations Justice, Science and Related Agencies subcommittee hearing. Rosenstein said due to federal regulations, Mueller can only be fired for good cause and that it must be put in writing why.