President Donald Trump evaluated new candidates to serve as his next chief of staff Sunday after plans for an orderly succession for departing John Kelly fell through.

The new hire was to be key to a West Wing reshuffling to shift focus toward the 2020 re-election campaign and the challenge of governing with Democrats in control of the House.

But even senior White House officials were caught off guard Sunday when Trump and Nick Ayers, whose hiring was believed to be a done deal, couldn't come to terms. No obvious successor was in sight and there was some fretting that Trump may not be able to fill the job by the time Kelly was set to leave around year's end.

Ayers, the chief of staff to Vice President Mike Pence, was seen as the favorite for the job when Trump announced Saturday that Kelly would step down. But a White House official said Sunday that Trump and Ayers could not reach agreement on Ayers' length of service and that he would instead assist the president from outside the administration.

Ayers confirmed the decision in a tweet, thanking Trump and Pence for giving him the opportunity to work in the White House. 'I will be departing at the end of the year but will work with the #MAGA team to advance the cause,' he said.

In a tweet of his own, Trump laid out the agenda: 'I am in the process of interviewing some really great people for the position of White House Chief of Staff. Fake News has been saying with certainty it was Nick Ayers, a spectacular person who will always be with our #MAGA agenda. I will be making a decision soon!'

With Ayers out of the running, Trump was considering four candidates for the post, including Office of Management and Budget director Mick Mulvaney, according to a person familiar with the president's thinking. Also emerging as a candidate was Representative Mark Meadows, a North Carolina Republican and the chairman of the House Freedom Caucus.

But Mulvaney was not interested in becoming chief of staff, according to a person close to him who spoke on condition of anonymity. Mulvaney has been saying for almost two months now that he would be more interested in becoming commerce or treasury secretary if that would be helpful to the president, the person said.

Also among those thought to be in the mix were Treasury Secretary Steve Mnuchin and US Trade Representative Robert Lighthizer, who said in a CBS interview that he hadn't spoken to anyone at the White House about the job and was 'entirely focused' on his position. A person familiar with Mnuchin's thinking said he, too, was happy with his work at Treasury and had not sought the job of chief of staff.

Acting Attorney General Matthew Whitaker and Trump's former deputy campaign manager, David Bossie, were also among the names being floated by those close to the White House.

Trump wants his next chief of staff to hold the job through the 2020 election, said the White House official, who was not authorized to discuss the personnel issue by name and, as did others, spoke on condition of anonymity.

Ayers and Trump had discussed the job for months. The father of young triplets, he had long planned to leave the administration at the end of the year, and had only agreed to serve in an interim basis through next spring.

Ayers will run a pro-Trump super PAC, according to a person familiar with his plans who was not authorized to discuss them by name.

Kelly, whose last day on the job is set to be Jan. 2, had been credited with imposing order on a chaotic West Wing after his arrival in June 2017 from his post as homeland security secretary. But his iron fist also alienated some longtime Trump allies, and over time he grew increasingly isolated, with an increasingly diminished role.