John McCain has called on conservatives within the US Republican Party to rally behind him after Mitt Romney suspended his campaign for the presidency.
The Arizona senator, who analysts say has effectively been handed the party's nomination, paid tribute to Mr Romney and invited his supporters to join him.
Mr Romney did not endorse Mr McCain, but spoke of the need for Republicans to unite around a single candidate.
The decision came after he fell well behind in the race on Super Tuesday.
Mr Romney said it had not been an easy choice for him or his family, but that he had decided that fighting on until the party's national convention in the summer would forestall the launch of a national campaign and help the Democrats.
"I entered this race because I love America, and because I love America in this time of war I feel I have to now stand aside for our party and for our country," he said.
Mr Romney said that while he disagreed with Mr McCain on a number of issues, they both agreed that the US needed to do whatever it took to be successful in Iraq and defeat terrorism.
The BBC's James Coomarasamy in Washington says that by acknowledging he had little chance of victory, Mr Romney will be hoping to emerge from a campaign which has cost him millions of dollars with kudos and a degree of leverage to win concessions from the eventual nominee.
Officially, former Baptist minister Mike Huckabee and Ron Paul are still in the campaign, but they stand no realistic chance of victory and may now come under pressure to step aside for the sake of party unity, our correspondent says.
The Democratic Party nomination contest is still being closely fought by senators Hillary Clinton and Barack Obama.
'Proud to be a conservative'
In a speech to a gathering of conservative activists in Washington DC, Mr McCain appealed to Republicans to support him and focus on November's election while the Democratic hopefuls contested the remaining primaries.
"I am proud to be a conservative," he told the Conservative Political Action Conference (CPAC). "I am proud, very proud, to have come to public office as a foot soldier in the Reagan revolution.
"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," he added.
"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."
Arizona, California, Connecticut, Delaware, Florida, Illinois, Missouri, New Jersey, New York, Oklahoma, South Carolina, New Hampshire
Alaska, Colorado, Maine, Massachusetts, Michigan, Minnesota, Montana, Nevada, North Dakota, Wyoming, Utah
Alabama, Arkansas, Georgia, Iowa, Tennessee, West Virginia
1,191 delegates needed for nomination. Source: AP (includes all kinds of delegates)
In an effort to assuage the fears of conservatives within his party, who have often attacked him for being too liberal, Mr McCain stressed his opposition to abortion, his support of the "surge" in Iraq, as well opposition to banning assault weapons.
He also called for making President George W Bush's tax cuts, which he strongly opposed in 2001 and 2003, permanent.
However, Mr McCain was booed by the crowd when he discussed the topic of illegal immigration, which has placed him at odds with many Republicans, especially conservative talk radio show hosts.
"We have had a few disagreements, and none of us will pretend that we won't continue to have a few," he said.
"But even in disagreement, especially in disagreement, I will seek the counsel of my fellow conservatives. If I am convinced my judgment is in error, I will correct it."
Mr McCain also took the opportunity to apologise for skipping the CPAC conference last year, which angered many of its members.
Saturday: Louisiana and Washington state (multi-party); Nebraska (Democratic); Kansas (Republican)
Sunday: Maine (Democratic)
Tuesday: Maryland, Virginia and Washington DC (multi-party)
"I was merely preoccupied with the business of trying to escape the distinction of pre-season front-runner for the Republican nomination," he said.
Mr McCain emerged as clear leader on Tuesday after enjoying wins the big states of California and New York, as well as Illinois, Oklahoma, New Jersey, Missouri, Connecticut, Delaware, and his home state of Arizona.
His new main challenger, Mr Huckabee, who is popular with evangelical Christians, took five states on Tuesday, backing up the widely held view that Mr McCain lacks support from conservatives.
Mr Huckabee will speak to the CPAC gathering on Saturday.
Both men, as well as Mr Paul, will be on ballots in the Republican Party primaries in Washington state, Kansas and Louisiana on Saturday.