WASHINGTON – President Donald Trump announced Friday that Mick Mulvaney, the administration's top budget official, will serve as acting White House chief of staff – taking over a daunting task of managing a president who doesn't like to be managed.

Mulvaney, 51, director of the Office of Management and Budget, will take over the high-profile and difficult job from John Kelly, who is expected to leave by the end of the year. The president said in a tweet that Mulvaney will serve as acting chief of staff.

"Mick has done an outstanding job while in the Administration," Trump said in a tweet on Friday, ending days of speculation about the position. "I look forward to working with him in this new capacity as we continue to MAKE AMERICA GREAT AGAIN!"

Mulvaney, a lawyer and former South Carolina congressman, will have to accommodate a boss who likes to stage events on a moment's notice, often overrides aides' advice, and makes policy and staff announcements by tweet.

Chris Whipple, author of "The Gatekeepers: How the White House Chiefs of Staff Define Every Presidency," said, "Trump is a president for whom focus and discipline are anathema" – yet those are the very qualities any chief of staff needs to instill in any administration.

"That's why it may be Mission Impossible," Whipple said.

A senior administration official, speaking on the condition of anonymity, said there is no "no time limit" to Mulvaney's appointment. It was not clear why the president named him to an "acting" position rather than simply as "chief of staff."

"That’s what the president wants," the official said.

The White House said that Mulvaney won't resign but his deputy, Russell Vought, will handle operations for OMB

"Mick Mulvaney will not resign from the Office Of Management and Budget, but will spend all of his time devoted to his role as the acting Chief Of Staff for the President," White House press secretary Sarah Sanders said in a statement. "Russ Vought will handle day to day operations and run OMB."

Some experts said they believed the shift within the administration left more questions.

"I think it means they couldn’t find a successor who was willing to stay through the 2020 election," said Kathryn Dunn Tenpas, a senior fellow at Brookings Institution who specializes in White House staffing.

"No reason to have an 'acting' placeholder if someone really wanted the job," she added.

More: Key moments from John Kelly's tenure

Trump and aides said for days that the president was close to a decision, with candidates including presidential counselor Kellyanne Conway, Mulvaney, and former campaign aide David Bossie.

"Really good ones," Trump said this week. "Terrific people. Mostly well known, but terrific people."

One candidate for the job – New Jersey Gov. Chris Christie – issued a statement Friday saying “now is not the right time for me or my family to undertake this serious assignment."

The selection process began Saturday after Trump announced that Kelly would be leaving. A day later, however, Trump's favorite for the job – Nick Ayers, chief of staff to Vice President Mike Pence – turned down the presidential job and said he planned to leave the administration instead.

The coming year will present new challenges for Mulvaney, given ongoing investigations of the administration, the Democratic takeover of the U.S. House, and Trump's tendency to chart his own course.

Just two of the outside pressures: Special counsel Robert Mueller continues to investigate Russian meddling in the 2016 election. And federal prosecutors in New York have implicated Trump in a scheme to pay hush money to women who claim to have had affairs with him, with the goal of keeping them quiet during election season and evading campaign finance laws.

Then there are the internal pressures that created problems for Trump's first two chiefs of staff, Reince Priebus and John Kelly.

Both tried to give structure to Trump's schedule and restrict access to him, officials said. But Trump used his cellphone to reach out to friends and informal advisers, allowing them to go around the chief of staff to make their cases directly to the president.

The results included sudden schedule or policy changes (and leaks to the news media).

Trump will be the first president to have three chiefs of staff in less than two years, assuming Mulvaney starts before the Jan. 20 anniversary of his 2017 inauguration.

"The chief of staff job is very difficult in a normal time," said David Cohen, political scientist professor at the University of Akron. "Being chief of staff to Donald Trump is extremely challenging ... An impossible job became even more impossible."

A good chief of staff tells presidents hard truths and things they don't want to hear, Cohen and other analysts said, but Trump has not always been receptive to that sort of thing.

Trump has told aides he wants someone with a more political bent, both to cope with the Democratic House and to help prepare for the president's 2020 re-election campaign.

"It's not enough to have just a 2020 campaign manager," Whipple said. The chief of staff should be "someone who can help you govern effectively."

Trump has pushed back on the idea that some people don't want the job, saying he has a wealth of candidates for what has been, traditionally, one of the most sought-after jobs in government.

Boris Epshteyn, former special assistant to Trump at the White House, said many are eager to take the job, and that Trump is "a great boss to work for and with."

The president "gives those working for him a lot of leeway," said Epshteyn, now chief political analyst for Sinclair Broadcast Group. "It's a job many people would be honored to have."

Previous White House chiefs of staff have gone on to bigger and better things:

Future Vice President Dick Cheney served as chief of staff for President Gerald Ford.

Future CIA director Leon Panetta worked for President Bill Clinton.

James Baker, chief of staff to President Ronald Reagan, went on to be Treasury secretary and secretary of State (before returning to the chief's job for his long-term patron, President George H.W. Bush).

Trump's men, Priebus and Kelly, probably haven't had their reputations enhanced by their White House service, analysts said, something their potential replacements will have to consider.

"You can't ignore what two chiefs of staff went through," Dunn Tenpas said.

She joked that Trump might as well change the title.

"He's not hiring a chief of staff in the traditional sense," she said. "Just call it 'aide to the president.'"