WASHINGTON (AP) — Mitt Romney stepped away from a third presidential run and reset the field in the battle for the 2016 Republican nomination.

The former Massachusetts governor's decision freed up vast quantities of political oxygen for the potential runs by other mainstream Republicans, especially former Florida Gov. Jeb Bush — son of one Republican president and brother of another. The broad but still unannounced field of possible candidates is jockeying to be the Republican who faces off against likely Democrat nominee Hillary Rodham Clinton — the former first lady, New York senator and secretary of state.

The election to pick a successor to President Barack Obama still is 21 months away and none of the potential candidates has said for certain they will make a run for the Republican nomination. Bush has taken initial steps toward a run, and so have New Jersey Gov. Chris Christie, Wisconsin Gov. Scott Walker and Florida Sen. Marco Rubio.

Romney had run for president twice before. Before claiming the nomination to oppose Obama in 2012, he lost a primary bid to Sen. John McCain in 2008. Obama rolled up big victories both times.

For now, polls show Bush as the front-runner. The mainstream Republican candidates will be hard-pressed to avoid being pushed too far to the right of the political spectrum in their bid to win the nomination. The series of state primary and caucus votes are dominated by the conservative activists and voters.

Romney's drift to the right during the primaries was seen as one reason for his loss to Obama in 2012.

Bush and Romney are deeply at odds over the future direction of the Republican party, which has been unable to appeal to black and Hispanic voters. In the 2012 campaign Romney alienated Hispanics in particular by saying U.S. immigration policy should encourage people in the country illegally to "self-deport."

That position opened a rift with Bush, whose wife is Mexican born.

After his 2012 loss, Romney had repeatedly said he was done with presidential politics. But three weeks ago Romney appeared to do an about-face, saying he was considering another run. He made calls to former fundraisers, staff members and supporters, and gave three public speeches in which he outlined his potential vision for another campaign — focusing this time on the plight of the middle class.

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.

It was an attempt to distance himself from 2012, when he was caught on video telling a room full of wealthy donors that 47 percent of Americans — middle- and low-income Americans — would vote for Obama no matter what he did. Americans "who are dependent upon government, who believe that they are victims, who believe the government has a responsibility to care for them, who believe that they are entitled to health care, to food, to housing, to you-name-it."

In the end, Romney's exit on Friday reflected the defection to Bush of several of his former major donors and a veteran staffer.

While likely boosting the chances of Bush, Christie, Walker and Rubio, Romney's departure leaves unchanged the more conservative side of the field, with a group of candidates that will likely include Kentucky Sen. Rand Paul, Texas Sen. Ted Cruz, former neurosurgeon Ben Carson and former Arkansas Gov. Mike Huckabee.

Rubio, a former Romney ally, took to Twitter minutes after Romney's announcement to promote his own political action committee.

"He certainly earned the right to consider running," Rubio said in a statement, "so I deeply respect his decision to give the next generation a chance to lead."

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Steven R. Hurst is AP international political writer.