Nine U.S. senators signed a letter on Friday calling for Attorney General Bill Barr to immediately resign after the Justice Department submitted a new sentencing recommendation for former Trump adviser Roger Stone this week, NBC reports.

What they're saying: "The interference in this case by you or other senior DOJ officials working under you is a clear violation of your duty to defend fair, impartial, and equal justice for all Americans."

"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 shocking actions taken by you or your senior staff to seek special protections for Mr. Stone make a mockery of your responsibilities to seek equal justice under the law and reveal that you are unfit to head the DOJ."

The senators also wrote it is "not credible" for the attorney general to claim he oversees the DOJ independently, as he suggested in an ABC News interview on Thursday.

The senators who signed the letter:

Sen. Elizabeth Warren (Mass.)

Sen. Richard Blumenthal (Conn.)

Sen. Ed Markey (Mass.)

Sen. Patty Murray (Wash.)

Sen. Jeff Merkley (Ore.)

Sen. Chris Van Hollen (Md.)

Sen. Ron Wyden (Ore.)

Sen. Bernie Sanders (Vt.)

Sen. Mazie Hirono (Hawaii)

Trump acknowledged in a tweet earlier this week that Barr personally intervened to overrule career prosecutors' sentencing recommendation for Stone.

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