“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 said in a statement Friday.
The California Democrat is situated at the center of the Russia inquiry as she sits on both the Senate intelligence and judiciary committees.
"I’m growing increasingly concerned that the president will attempt to fire not only Robert Mueller, the special counsel investigating possible obstruction of justice, but also Deputy Attorney General [Rod] Rosenstein who appointed Mueller," Feinstein said.
Trump has posted a number of tweets calling the Russia probe a “witch hunt” being carried out by “very bad people.”
In a tweet Friday morning, Trump acknowledged for the first time that he was “being investigated for firing the FBI director by the man who told me to fire the FBI Director! Witch Hunt."
Trump was likely referring to Rosenstein, and to a Washington Post story that Mueller is looking at whether the president attempted to obstruct justice.
Rosenstein authored a memo in May that the White House initially used as justification for firing then-FBI Director James Comey, who was investigating allegations of Russian interference in the US presidential election.
The deputy attorney general later announced the selection of Mueller, a former FBI director, as special counsel to head up the investigation.
Sen. Feinstein stressed that it was Rosenstein, not the president, who has the authority to fire Mueller. Rosenstein testified this week that he would not fire Mueller without good cause after a longtime Trump ally raised the possibility that the president might dismiss the special counsel.
“If the president thinks he can fire Deputy Atty. Gen. 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," Feinstein said.
Inside the White House, those close to the president say he is angry at Justice Department officials, especially at Rosenstein for appointing Mueller.
Trump has vehemently denied any collusion between his campaign team and Russia.