By John Whitesides, Political Correspondent
McCain assured a conference of conservative activists he was one of them, citing his commitment to win in Iraq, halt Iran's nuclear ambitions and rein in the federal government while drawing sharp contrasts with potential Democratic opponents Hillary Clinton or Barack Obama.
"I feel I have to now stand aside, for our party and for our country," the former Massachusetts governor told the shocked crowd, some of whom gasped and shouted "No, no!" in response.
McCain, the 71-year-old former Vietnam prisoner of war, has become a target of critics on the right for his moderate views on illegal immigration, his votes against President George W. Bush's tax cuts in 2001 and 2003 and his labelling in 2000 of some religious conservative leaders as "agents of intolerance."
"And I am acutely aware that I cannot succeed in that endeavour, nor can our party prevail over the challenge we will face from either Senator Clinton or Senator Obama, without the support of dedicated conservatives," he said.
Former Arkansas Gov. Mike Huckabee, who won five states on Tuesday, remains in the race but will have a difficult task overcoming McCain, who has rolled up more than 700 of the 1,081 delegates needed to win the nomination.
McCain's name was booed by some members of the audience when Romney mentioned him, but he drew mostly cheers when he appeared before the crowd -- many of them McCain supporters brought in by the campaign.
He earned boos, however, when he brought up his support during last year's Senate debate for a path to citizenship for illegal immigrants. McCain has since said border security must come first.
Some attendees said they were disappointed Romney was leaving, calling him the only conservative candidate in the race.
Romney said he was pulling out of the race in order to let Republicans prepare for a general election battle against the two remaining Democrats, both whom have campaigned to withdraw U.S. troops from Iraq.
McCain said Obama, an Illinois senator, and Clinton, a New York senator, would create a disaster in Iraq, fail to recognize the threat posed by Iran and weaken the fight against Islamic extremists.
Clinton and Obama, locked in a tight and costly duel for the Democratic nomination, both announced new fundraising achievements and took note of McCain's ascension to the role of likely nominee.
Clinton, who said on Wednesday she had pumped $5 million (2.6 million pounds) of her own money into the race to keep up with Obama, raised $4 million online in the day after polls closed on Tuesday and a total of $7.5 million in the month, aides said.
(To read more about the U.S. political campaign, visit Reuters "Tales from the Trail: 2008" online at http://blogs.reuters.com/trail08/
