WASHINGTON — President Trump said on Monday that Attorney General William P. Barr would depart next week, ending a tenure marked by Mr. Barr’s willingness to advance the president’s political agenda and by criticism that he eroded the post-Watergate independence of the Justice Department.

Mr. Barr had in recent weeks fallen out of favor with the president after acknowledging that the department had found no widespread voter fraud. Tensions between them escalated this past weekend when Mr. Trump accused his attorney general of disloyalty for not publicly disclosing the department’s investigation into President-elect Joseph R. Biden Jr.’s son Hunter Biden during the campaign.

Mr. Trump sought to play down their differences on Monday, saying in a tweet announcing Mr. Barr’s departure that “our relationship has been a very good one, he has done an outstanding job!” The decision to quit was Mr. Barr’s, not the president’s, a person familiar with the matter said, and the White House counsel, Pat A. Cipollone, helped to facilitate his exit on cordial terms with Mr. Trump.

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Mr. Barr, 70, who also served as attorney general in the George Bush administration, was viewed initially in Washington as a possible stabilizing force in the chaotic Trump era, but that expectation dissipated as he took aim at the Justice Department’s own investigation into the Trump campaign’s ties to Russia that had long antagonized the president.

Mr. Barr brought the Justice Department closer to the White House than any attorney general in a half-century. Defying the distance that federal law enforcement officials have typically maintained from campaign politics, Mr. Barr spent the months leading up to the election echoing Mr. Trump’s unsubstantiated claims of widespread voter fraud. He also told an interviewer that the country would be “irrevocably committed to the socialist path” if the president were not re-elected.

But Mr. Barr backed off the warnings of voter fraud after the election, saying little publicly for weeks until he said that the department had received no evidence that would overturn Mr. Biden’s election. “To date, we have not seen fraud on a scale that could have effected a different outcome in the election,” Mr. Barr told The Associated Press.