(Adds comments from two experts on chiefs of staff and additional background)

By Roberta Rampton and Steve Holland

WASHINGTON, Dec 14 (Reuters) - U.S. President Donald Trump on Friday moved to end speculation that he was having a hard time finding a new chief of staff, naming his budget chief Mick Mulvaney to the top post on a temporary basis.

Mulvaney, a hard-charging conservative and former congressman, will be the third person in two years to try to bring order to what has often been a chaotic White House. Trump named him after two other prominent candidates withdrew from consideration in the space of a week.

"For the record, there were MANY people who wanted to be the White House Chief of Staff," Trump said on Twitter. "Mick M will do a GREAT job!"

The job is seen as one of the most important jobs in Washington: the gatekeeper to the president charged with marshalling the resources of the office to carry out his priorities.

The most recent occupant - John Kelly, a retired Marine Corps general - had some success in restoring order after he was appointed in July 2017, but had a rocky relationship with Trump. Kelly will stay through the end of December.

Mulvaney will take the reins at a time when Republican Trump, weakened by Democrats winning control of the U.S. House of Representatives in elections last month, grapples with investigations into his businesses and most controversial policies. In addition, Special Counsel Robert Mueller is investigating whether there was collusion between Trump's 2016 election campaign team and Russian officials.

Mulvaney, 51, rose to prominence as a founder of the powerful House Freedom Caucus conservative voting bloc. He brings an in-depth knowledge of Congress to the job.

As head of the Office of Management and Budget (OMB), he has experience working with Trump and his top aides at the White House.

But the temporary nature of Mulvaney's appointment may undermine his authority in speaking for the president, and limit his ability to do the job, said Chris Whipple, author of a book on White House chiefs of staff called "The Gatekeepers."

"Donald Trump desperately needs a White House chief who can execute his agenda, and you can't do that if you have an expiration date on your forehead," Whipple said.

'CONTINUED AMBIGUITY'

Mulvaney, who has often defended Trump's economic policies on the closely watched Sunday political talk shows, has long been cited as a contender for the chief of staff job.

He has led budget talks with the U.S. Congress for Trump, including during a government shutdown in January. In 2013, with the House Freedom Caucus, Mulvaney helped orchestrate a government shutdown as a protest against funding for then-President Barack Obama's healthcare law.