NEW YORK -- Mitt Romney ended his rollercoaster return to presidential politics on Friday, declaring his party would be better served by the "next generation of Republican leaders" and concluding his unlikely comeback as suddenly as it began.
Aides said it was a deeply personal and even painful decision for the 2012 Republican presidential nominee. He insisted he could win the next election if he ran, but his announcement followed a three-week fact-finding effort that revealed significant resistance to a third campaign.
"I believe that one of our next generation of Republican leaders, one who may not be as well-known as I am today, one who has not yet taken their message across the country, one who is just getting started, may well emerge as being better able to defeat the Democrat nominee," Romney told supporters on a conference call. "In fact, I expect and hope that to be the case."
The remark was both a recognition of his own limitations and an indirect swipe at the man who created the urgency behind Romney's brief flirtation with a third presidential campaign. That is former Florida Gov. Jeb Bush, the son and brother of former presidents, who is speeding toward a campaign of his own.
Bush and New Jersey Gov. Chris Christie would have served as Romney's most likely rivals for the support of the GOP establishment, and both men felt an immediate impact. The announcement sparked a rush of activity by Romney loyalists - operatives and donors alike - suddenly freed to support another White House hopeful as the crowded 2016 field begins to take shape.
Devoted Romney supporter Bill Kunkler, part of Chicago's wealthy Crown family, said he was disappointed by Friday's news but now was all-in for Bush.
"I'll work for Jeb. Period. And no one else," Kunkler said, noting that he planned to attend a Feb. 18 Chicago fundraiser for Bush hosted by former Romney backers.
Bobbie Kilberg, a top GOP fundraiser based in Virginia, quickly settled on Christie.
"We had long and deep ties and friendship with Mitt," she said. "That has changed obviously, at 11 o'clock this morning."
Romney's aides insist there was no specific incident that caused Friday's abrupt announcement, which came during a late morning conference call with close supporters and former staffers.
The former Massachusetts governor, who is 67, shocked the political world three weeks earlier, when signaled interest in a third presidential run during a private meeting with former donors in New York.
That followed what aides describe as several months of strong encouragement from Republicans as he toured the country raising money and energy for GOP colleagues.
"No one asked McCain to run again," said longtime Romney aide Ron Kaufman, a reference to 2008 nominee John McCain. "Thousands of people asked Mitt to run again."
The surprise announcement of Romney's interest three weeks ago in the office of New York Jets owner Woody Johnson was the first public step in a fact-finding mission meant to assess the 2016 outlook. Romney, a longtime business executive, has typically followed a scientific approach to challenges - political and otherwise - and demanded data before making a decision.
He and his most trusted advisers plunged into phone calls and personal visits with key GOP officials and activists across the country.
At the same time, Romney tested a new stump speech focused on the poor and middle class in three public appearances. Critics jabbed the new focus as an insincere shift designed to shed his image as an out-of-touch millionaire. Those closer to Romney suggested it was a truer reflection of a man of deep faith than most voters saw during his first two presidential campaigns.
The evaluation phase peaked during a gathering of senior aides one week ago at the Boston offices of Solamere Capital, an investment firm led by his eldest son, Tagg Romney, and top fundraiser, Spencer Zwick.
Aides offered Romney a blunt assessment of his 2016 prospects, suggesting there was still a path to victory but also signs of eroding support among donors and in former strongholds such as New Hampshire. They made clear that a new bid for the GOP nomination would be more challenging than his second, when Romney dominated a field that never featured another strong establishment alternative such as Bush or Christie.
In the subsequent days, several major Romney donors and one of his most trusted veteran staffers - someone who had participated in the Boston meeting - defected to Bush's team. The trend was unmistakable, despite Romney's optimism.
The Friday conference call ended what was always intended to be a brief trial period.
"I am convinced that we could win the nomination, but I fully realize it would have been difficult test and a hard fight," Romney said.
He was having dinner Friday night with Christie, who was among his staunchest backers during the 2012 race. Romney is not, however, expected to endorse another Republican candidate in the near future.
And he left the door open, if only a crack, to another comeback. He said he had been asked if there were any circumstance under which he would again reconsider.
That, he said, "seems unlikely."
Mitt Romney says he will not run for president in 2016