Barr is using the Justice Department to further Trump's personal and political interests, the letter said.

If anyone else who is not a friend of the president "were to lie to federal investigators in the course of a properly predicated counterintelligence investigation, and admit we did so under oath, we could be prosecuted," the letter said.

The letter calls on Barr to resign and encourages Congress to formally censure Barr over "his repeated assaults on the rule of law in doing the President’s personal bidding rather than acting in the public interest."

Barr directed federal prosecutors to abandon their prosecution of Michael Flynn, who served briefly as national security adviser in the early days of the Trump administration. Flynn admitted that he had lied to the FBI about his conversations during the transition with Russia's ambassador to the U.S.