WASHINGTON — Mick Mulvaney will serve as acting White House chief of staff, President Donald Trump said Friday.

Mulvaney, the director of the White House Office of Management and Budget and a former Republican congressman from South Carolina, will take over for chief of staff John Kelly, who is due to leave at the end of the year.

....I look forward to working with him in this new capacity as we continue to MAKE AMERICA GREAT AGAIN! John will be staying until the end of the year. He is a GREAT PATRIOT and I want to personally thank him for his service! — Donald J. Trump (@realDonaldTrump) December 14, 2018

Trump announced the decision on Friday, hours after Chris Christie, the former New Jersey governor, joined Rep. Mark Meadows, R-N.C., and Nick Ayers, the top aide to Vice President Mike Pence, in taking himself out of the running.

"He got picked because the president likes him, they get along," White House Communications Director Bill Shine said. "Former member of Congress, he knows Congress, Capitol Hill."

Mulvaney, who has been seen as a steady hand within the administration, will be taking on a heavy caseload of political, legal and policy problems.

One of the conservatives who helped launch the House Freedom Caucus, a band of lawmakers that frequently votes against GOP leadership on budget matters, Mulvaney's elevation comes at a time when Trump and congressional Democrats are at loggerheads over whether to include funding for the president's promised wall along the U.S. border with Mexico in a year-end spending agreement.

If they can't strike a deal on either a stopgap measure or a longer-term funding bill by Dec. 21, parts of the federal government will shut down.

This is a tremendous honor. I look forward to working with the President and the entire team. It’s going to be a great 2019! https://t.co/X4gauBOg1d — Mick Mulvaney (@MickMulvaneyOMB) December 14, 2018

Mulvaney, who has also served as acting head of the Consumer Financial Protection Bureau, has taken on tough tasks for the administration without drawing the president's ire — a rare feat among his aides, advisers and Cabinet officials.

His budgets have called for sharp cuts in domestic spending and foreign aid that have largely been rejected by Congress, and, after tax cuts and a big boost in military spending backed by the administration, the Congressional Budget Office projects that the federal deficit will rise over $1 trillion in the fiscal year that begins Oct. 1.

The Senate confirmed Kathy Kraninger, Mulvaney's replacement at the CFPB, last week.

Mulvaney will technically remain director of OMB, but his deputy, Russ Vought, will run the agency on a day-to-day basis, said White House press secretary Sarah Sanders.

If Mulvaney gave up the OMB post, which requires Senate confirmation, he would have to go through the nomination process to reclaim it in the future. But by retaining it, there potentially would be a soft landing open to him if the president chooses to hire another chief of staff in the future.

Trump did not announce a time limit on Mulvaney's tenure as acting chief of staff, and two senior White House officials dismissed the idea that the president would continue his search for a permanent chief of staff.

On Thursday night, Christie, who campaigned for Trump in 2016 and was later pushed out as chief of his presidential transition team, had spent an hour discussing the job with the president at the White House residence, according to two sources familiar with the chief-of-staff-selection process.

The unusual spectacle of the president so publicly courting and being rebuffed by candidates for what is typically one of the most sought-after jobs in government comes at a time when Trump is battling special counsel Robert Mueller's investigation, separate allegations that he conspired to break campaign finance laws and his party's loss of power in the House.