ATLANTA (AP) — The GOP's crowded 2016 presidential field has shifted sharply, with Paul Ryan, the party's last vice presidential nominee, bowing out, two-time presidential hopeful Mitt Romney marching toward his third campaign and New Jersey Gov. Chris Christie stepping closer to his first.

The developments came as Republican officials from across the nation prepare to gather in San Diego this week to finalize an election calendar that will include the first debates of the campaign this summer. Romney is among four potential White House contenders expected to attend that meeting.

Ryan, a Wisconsin congressman and close Romney ally, thanked those who encouraged him to seek the presidency, but said he wanted to focus all of his attention on his new role as chairman of the House Ways and Means Committee.

"I'm ready to get on with my job, and I'm excited about what's ahead," Ryan told The Associated Press in an interview at the Capitol.

Ryan was among those Romney personally telephoned over the weekend to gauge their support for a third campaign, which was barely on the horizon at this time last week and would be the first return engagement of a losing major party nominee in decades.

Asked Monday if he would back another Romney run, Ryan demurred, saying, "It's premature for any of that."

The former Massachusetts governor shocked former staff, supporters and donors on Friday when he told a private gathering in New York that he is again seriously exploring a campaign, a change for Romney after several months of insisting his career in politics had comes to its end.

The next day, he and his wife, Ann, began reaching out to others as members of their inner circle started to revive a political operation that had begun to move on.

"The chances of him running are better than 50-50," said former Minnesota Gov. Tim Pawlenty, who spoke with Romney over the weekend.

Calls from Romney went to several other boldface names in Republican politics, including Ryan; former House Speaker Newt Gingrich; New Hampshire Sen. Kelly Ayotte and the state's former governor, John Sununu; Meg Whitman, chief executive at Hewlett-Packard Co.; Utah Rep. Jason Chaffetz; two former senators, Scott Brown of Massachusetts and Jim Talent of Missouri; and David Kochel, Romney's senior Iowa strategist in both of his previous campaigns.

"We had a great conversation. He made it clear to me he is seriously considering the race," said Jim Merrill, a top Romney staffer in New Hampshire during the 2008 and 2012 campaigns. "When you get a phone call like that at close to 9 on a Saturday night, it means something."

Former aides suggested Monday that Romney could easily become a force in the coming contest, pointing to the more than $1 billion he raised and 60 million votes he garnered in the last election. Yet he would face far greater challenges in the next presidential contest than he did in the last, particularly in the early primaries against a field expected to feature more than a dozen Republicans with legitimate White House resumes, including several sitting governors and U.S. senators.

None threatens Romney's prospects more than former Florida Gov. Jeb Bush, an early establishment favorite who has already begun to draw from the same pool of Republican donors who fueled Romney's last campaign.

Bush has yet to formally launch a campaign, but has been among the most aggressive would-be candidates in recent weeks, having launched a fundraising operation and attended a series of private meetings with donors across the country.

Romney could also face another establishment favorite, Christie, who is sending new signals that an announcement about his presidential plans could be imminent.

The Republican National Committee's top fundraiser, Texas-based Ray Washburne, is preparing to join Christie's team. He told the AP on Monday, "I won't be the national finance chairman at the end of the week." He declined to comment further because Christie has yet to announce his 2016 intentions.

Washburne was a leading fundraiser for Bush's brother, former President George W. Bush.

Christie's team is expected to start making moves to raise money for a prospective presidential campaign by the end of the month, according to a person who has been in touch with Christie's team, but who spoke on the condition of anonymity because the person was not authorized to discuss the plans publicly.

That could include the potential formation of a political action committee to begin raising cash that can be used to support a prospective campaign, the person said.

Christie's chief political adviser, Mike DuHaime, declined to comment.

Meanwhile, a handful of presidential contenders were preparing Monday to appear later this week at the Republican National Committee's winter meeting in San Diego. The agenda is expected to include Romney, former neurosurgeon and conservative favorite Ben Carson, Wisconsin Gov. Scott Walker and outgoing Texas Gov. Rick Perry.

The gathering will take place close to Romney's southern California home.

In New Hampshire on Monday, former New York Gov. George Pataki capped a two-day trip to the early voting state sounding very much like a politician ready to jump into the race. He told the AP in an interview he's again considering a White House campaign because the nation can't risk electing another Democratic president.

"That's one of the reasons I think the outcome of this election is so important, and I'm so much more inclined to get involved," Pataki said.

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