WASHINGTON (AP) — Attorney General William Barr, one of President Donald Trump’s staunchest allies, is departing amid lingering tension over the president’s claims of election fraud and the investigation into President-elect Joe Biden’s son.

Barr went Monday to the White House, where Trump said the attorney general submitted his letter of resignation. “As per letter, Bill will be leaving just before Christmas to spend the holidays with his family,” Trump tweeted.

Trump has publicly expressed his anger about Barr’s statement to The Associated Press earlier this month that the Justice Department had found no widespread fraud that would change the outcome of the election. Trump has also been angry that the Justice Department did not publicly announce it was investigating Hunter Biden ahead of the election, despite department policy against such a pronouncement.

Barr told the AP that U.S. attorneys and FBI agents have been working to follow up specific complaints and information they’ve received, but “to date, we have not seen fraud on a scale that could have effected a different outcome in the election.”

Barr’s resignation leaves Trump without a critical ally as he winds down his final weeks in office, and it throws into question open Justice Department investigations, especially the probe into Hunter Biden’s taxes.

In his resignation letter, Barr said he updated Trump Monday on the department’s “review of voter fraud allegations in the 2020 election and how these allegations will continue to be pursued.” He added that his last day on the job would be Dec. 23.

Trump said Deputy Attorney General Jeff Rosen, whom he labeled “an outstanding person,” will become acting attorney general. As the current second in command at the Justice Department, Rosen’s appointment is not likely to change much in the final weeks before the administration departs.