A group of Democratic senators wrote a letter to Attorney General William Barr Friday calling on him to resign after Justice Department officials intervened in the case against Trump associate Roger Stone. The senators said that it is "not credible" for Barr to claim he oversees the Justice Department in an independent manner, as he claimed in an interview with ABC News on Thursday.

Senators Richard Blumenthal, Ed Markey, Patty Murray, Jeff Merkley, Chris Van Hollen, Ron Wyden, Bernie Sanders and Mazie Hirono signed the letter "to express our alarm and opposition to the unethical political intervention" of Justice Department officials in Stone's case.

"This is an extraordinary turn of events. It appears to show that you and other top DOJ officials intervened in a clearly political fashion to undermine the administration of justice at the President's behest in order to protect a well-connected political ally who committed a 'direct and brazen attack on the rule of law,'" the senators wrote in their letter.

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On Monday, four federal prosecutors recommended that Stone receive seven to nine years in prison. Mr. Trump wrote a tweet that night condemning the sentence as overly punitive. Justice Department officials intervened to ask for a more lenient sentence for Stone on Tuesday, and the prosecutors all withdrew from the case. Mr. Trump thanked Barr in a tweet on Wednesday for "taking charge of a case that was totally out of control and perhaps should not have even been brought."

"It demonstrates that you lied to Congress during your confirmation hearing when you stated that you would 'keep the enforcement process sacrosanct from political influence,' and it reveals your unwillingness or inability to maintain the integrity of the DOJ and to uphold justice and the rule of law," the senators said about the apparent intervention.

In the interview with ABC News, Barr said the president "has never asked me to do anything in a criminal case" but should stop tweeting about the Justice Department because public statements and tweets about people at the Justice Department and pending cases "make it impossible for me to do my job and to assure the courts and the prosecutors in the department that we're doing our work with integrity."

Barr also said that he was "not going to be bullied or influenced by anybody" as the leader of the Justice Department. But the senators said this assertion "is simply not credible given that it is sharply at odds with the behavior of top DOJ officials and the comments of the President over the past 72 hours."

Warren, Van Hollen, Hirono and Markey are also co-sponsoring a bill to prohibit Trump Justice Department appointees from participating in matters relating to the president, his family, or his associates.

"Attorney General Barr's interference in Roger Stone's sentencing is not just unethical-it's corrupt, plain and simple," Warren said in a statement released alongside the legislation announcement. "This bill would use Congress' spending authority to protect the rule of law and prevent a corrupt attorney general from protecting the president's buddies when they commit crimes to benefit the president."

Kathryn Watson contributed to this report