The two-time Republican candidate told a group of donors on a conference call simulcast on CNN that he has "decided it is best to give other leaders in the party the opportunity to become our next nominee."
"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 said. "In fact, I expect and hope that to be the case."
Romney, who lost the Republican nomination to Sen. John McCain (R-Ariz.) in 2008 and the general election to President Barack Obama in 2012, had teased supporters with increasingly pointed hints that he was ready to hit the trail one more time.
That decision would have represented a dramatic change of heart for the former Massachusetts governor.
Asked by the New York Times last year if he'd ever consider running again, Romney said, "Oh, no, no, no. No, no, no, no, no. No, no, no. People are always gracious and say, 'Oh, you should run again.' I'm not running again. I will say this: It was a great experience. I loved it."
But on Jan. 17, Romney told a group of Republican National Committee members at a winter meeting in California, "Me, I'm giving some serious consideration to my future." Pointing to his wife, Ann, he said, "She believes that people get better with experience, and heaven knows I have experience running for president."
Romney insiders, donors, friends and associates had been goosing reporters incessantly for the past couple of months, insisting that the question of whether he would run was very real and the decision incredibly difficult.
