Updated at 5:53 p.m. ET

President Trump said Friday evening that Mick Mulvaney, his director of the Office of Management and Budget, will be the acting White House chief of staff.

It's unclear how long Mulvaney will serve in the role, succeeding outgoing chief of staff John Kelly. Trump announced on Dec. 8 that Kelly would leave at the end of the year.

"There's no time limit. [Mulvaney's] the acting chief of staff, which means he's the chief of staff. He got picked because the president liked him they get along," a senior administration official told reporters at the White House. When asked why Mulvaney was named as "acting" rather than the permanent chief of staff, the official responded, "Because that's what the president wants."

Vice President Pence's chief of staff, Nick Ayers, had been the leading candidate to replace Kelly, but he pulled out of consideration. Former New Jersey Gov. Chris Christie also said in a statement Friday he wasn't interested in the job.

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