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Former Massachusetts Governor Mitt Romney has suspended his campaign for the Republican party nomination for the US presidency.
Mr Romney spent millions of dollars of his own money on the campaign, but fell well behind front-runner John McCain after Super Tuesday's primaries.
Correspondents say his exit has in effect cleared the way for Senator McCain as Republican candidate.
Senator McCain congratulated Mr Romney and invited his supporters to join him.
"You are welcome to join my campaign, and it will be a campaign based on conservative principles and a consequential election about the country's future," he said.
"I'd be deeply humbled to receive the nomination of my party as we continue to move forward in this campaign."
I am convinced that unless America changes course, we will become the France of the 21st Century - still a great nation, but no longer the leader of the world
Why Mitt Romney quit
Profile: Mitt Romney
Mitt quits in presidential style
The BBC's Kevin Connolly in Washington says Senator McCain worked hard to reach out to conservative Republicans, who have frequently attacked him for being too liberal.
He stressed his support for core issues such the pro-life movement and permanent tax cuts, as well as President Bush's troop surge in Iraq.
Officially former Baptist minister Mike Huckabee and Ron Paul are still in the campaign, but they stand no realistic chance of victory.
Analysts, however, say Mr Romney has not officially dropped out of the race. Suspending the campaign allows him to lay claim to his delegates and use them as leverage to win concessions from the eventual nominee.
He announced his decision at the Conservative Political Action Conference.
He said the decision had not been an easy one.
12 states, 707 delegates
Arizona, California, Connecticut, Delaware, Florida, Illinois, Missouri, New Jersey, New York, Oklahoma, South Carolina, New Hampshire
11 states, 294 delegates
Alaska, Colorado, Maine, Massachusetts, Michigan, Minnesota, Montana, Nevada, North Dakota, Wyoming, Utah
6 states, 195 delegates
Alabama, Arkansas, Georgia, Iowa, Tennessee, West Virginia
1,191 delegates needed for nomination. Source: AP (includes all kinds of delegates)
Excerpts: Romney speech
"If I fight on in my campaign, all the way to the convention, I would forestall the launch of a national campaign and frankly I'd be making it easier for Senator Clinton or [Barack] Obama to win," Mr Romney said at the conference.
He added that he would continue to fight for conservative principles, which were now needed more than ever.
"I am convinced that unless America changes course, we will become the France of the 21st Century - still a great nation, but no longer the leader of the world, no longer the superpower," he said.
A successful businessman, he had hoped to be the first US president from the Mormon religion.
But he failed to translate leads in opinion polls into victories in the early primary states of Iowa and New Hampshire.
In recent weeks, conservatives in the party rallied behind him as a candidate who they hoped could stop Senator McCain.
But the BBC's Justin Webb in Washington says he faced resistance among some evangelical Christians because of his Mormon faith.
He also had difficulty convincing his party that he held strong beliefs on some very important subjects, such as abortion rights, immigration and gun ownership.
'Unite the party'
Mr McCain skipped the annual conference last year, angering many members of the group.
Saturday: Louisiana and Washington state (multi-party); Nebraska (Democratic); Kansas (Republican)
Sunday: Maine (Democratic)
Tuesday: Maryland, Virginia and Washington DC (multi-party)
Q: What's next?
He enjoyed wins in the big states of California and New York, as well as Illinois, Oklahoma, New Jersey, Missouri, Connecticut, Delaware, and his home state of Arizona.
Some key conservative figures have refused to vote for Mr McCain in the presidential election if he wins the nomination.
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 in his own party.
Mr Huckabee will speak to the conservative conference on Saturday.