Former Massachusetts Governor Mitt Romney has withdrawn from the race for the Republican party nomination for the US presidency.
Mitt Romney announcing the withdrawal of his White House bid.
"I feel I have to now stand aside, for our party and for our country," he said in a speech to the Conservative Political Action Conference.
Mr Romney lost 14 of 21 states on Super Tuesday, the biggest day of US presidential voting ahead of November's election, while Senator McCain savored coast-to-coast wins and cemented his position as front-runner.
The former Massachusetts governor had vowed to fight on after Super Tuesday, but he and his campaign advisers discussed the situation on Wednesday. Romney has been helping to bankroll his campaign using his own personal wealth.
"This is not an easy decision for me. I hate to lose," Mr Romney said.
Many Republicans are eager for an end to the nominating contest between McCain, Romney and former Arkansas Gov. Mike Huckabee in order to begin what is expected to be a difficult fight against either of the two Democratic candidates, Hillary Clinton or Barack Obama.
Mr Romney's decision will raise pressure on Mr Huckabee to do the same.
"If I fight on in my campaign, all the way to the convention, I would forestall the launch of a national campaign and make it more likely that Sens. Clinton or Obama would win," Mr Romney said.
Mr Romney did not endorse Mr McCain in his speech. The two have engaged in a bitter crossfire in recent weeks over who is the real conservative.
Mr Romney said, "I disagree with Senator McCain on a number of issues, as you know."
But he said he agreed with him "on doing whatever it takes to be successful in Iraq, on finding and executing Osama bin Laden, and on eliminating al Qaeda and terror."
The McCain campaign declined immediate comment. McCain was to address the conference after Mr Romney.