ATLANTA -- Former Republican presidential candidate Mitt Romney and his wife reached out to former staff and supporters in key states over the weekend, making contact just hours after shocking many by signalling interest in a third run for the White House.

Romney, who ran unsuccessfully against President Barack Obama in 2012, told a private gathering of New York-area donors on Friday that he is again seriously exploring running for the White House. The next day, Romney began phoning former backers in Iowa and New Hampshire, the two states that kick off the drawn-out, state-by-state primary process for choosing a nominee.

Romney was the overwhelming front-runner for the Republican nomination at the beginning of the 2012 campaign. This time, he would join a 2016 field expected to feature more than a dozen Republicans with legitimate White House potential, including several sitting governors and U.S. senators.

None threatens Romney's prospects more than former Florida Gov. Jeb Bush, and early establishment favourite 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.

The Democratic field, in contrast, has a clear early front-runner in former Secretary of State Hillary Rodham Clinton, even though she has not formally declared her intention to run.

Jim Merrill, a top Romney staffer in New Hampshire during his 2008 and 2012 campaigns, said he spoke to Romney over the weekend.

"We had a great conversation. He made it clear to me he is seriously considering the race," Merrill said in an interview with The Associated Press.

Calls from Romney also went to several boldface names in Republican politics, including his 2012 running mate, Wisconsin Rep. Paul Ryan, who said Monday he would not run for president in 2016.

Ryan, who had been the focus of much speculation about his 2016 ambitions, said he wanted to concentrate on his work as chairman of a powerful tax-writing committee in Congress. He said it was "premature" to endorse Romney or any other potential candidate.

Romney's move toward the 2016 campaign comes after months on insistence that he would not seek the White House for a third time.

With little indication that Romney was seriously contemplating a run, many of his former staffers and donors have already moved on to other potential candidates.

The Republican National Committee's top fundraiser is stepping down this week as he prepares to join New Jersey Gov. Chris Christie's prospective presidential campaign. Christie is also for some of the same establishment-minded donors as Romney and Bush.

Former New York Gov. George Pataki, a Republican, said Monday he is also seriously considering a White House campaign, telling AP that the U.S. can't risk electing another Democratic president. Pataki, who left office in 2007 after serving three terms as governor of New York, spent two days in New Hampshire and said he was building a campaign team and starting a fundraising operation.

Romney also called a handful of others supporters in New Hampshire, while his wife, Ann Romney, phoned Susan Duprey, her personal travelling companion during the 2012 campaign and the wife of New Hampshire's national committeeman, Steve Duprey.

The developments came as Republican officials from across the U.S. prepare to gather in San Diego this week to finalize an election calendar that will include the first debates of the campaign this summer.