Republican John McCain asked disgruntled conservatives to support his presidential bid on Thursday, shortly after Mitt Romney ended his struggling campaign and made McCain the all-but-certain nominee.

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.

McCain's speech, which drew boos on the topic of illegal immigration, followed by a few hours Romney's surprise announcement at the conference that he was ending his run to allow Republicans to focus on the November election.

"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, who has built an almost insurmountable lead in delegates to the party's nominating convention, pleaded for party unity during his appearance at the annual conference.

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."

"I know I have a responsibility, if I am, as I hope to be, the Republican nominee for president, to unite the party and prepare for the great contest in November," the Arizona senator told the activists gathered in a Washington hotel.

"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.

Romney pulled out after losing 14 of 21 states on Tuesday, the biggest day of U.S. presidential voting ahead of November's election, while McCain romped to coast-to-coast wins and cemented his position as front-runner.

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.

"This is a two-man race for the nomination, and I am committed to marching on," Huckabee said in a statement after Romney's withdrawal.

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.

SOME BOOS

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.

"It is my sincere hope that even if you believe I have occasionally erred in my reasoning as a fellow conservative, you will still allow that I have, in many ways important to all of us, maintained the record of a conservative," McCain told the conference.

Some attendees said they were disappointed Romney was leaving, calling him the only conservative candidate in the race.

"This leaves me very concerned about the future of the Republican Party," said Nathan Shapiro, 22, a college student in New York. "I don't think McCain will carry on the traditions of the Republican Party, he's not a real conservative."

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.

"In this time of war, I simply cannot let my campaign be a part of aiding a surrender to terror," he said.

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.

"Their resolve to combat it will be as flawed as their judgment," he said.

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.

Obama agreed to hold two more debates with Clinton before the March 4 nominating contests, his campaign said. Clinton had pushed for five debates in that period.

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.

Obama raised more than $7 million since polls closed on Tuesday, his camp said.