By Steve Holland
"I feel I have to now stand aside, for our party and for our country," the former Massachusetts governor said in a speech to the Conservative Political Action Conference.
The writing was on the wall for Romney after he lost 14 of 21 states on Tuesday, the biggest day of U.S. presidential voting ahead of November's election, while Arizona Sen. McCain romped to coast-to-coast wins and cemented his position as front-runner.
McCain's named was booed by some members of the audience at the conference when Romney mentioned him.
"In this time of war, I simply cannot let my campaign be a part of aiding a surrender to terror," he said.
"This is not an easy decision for me. I hate to lose," an emotional-looking Romney said.
Romney's decision will raise pressure on Huckabee to do the same, although he has said he will continue his run that has been fuelled by evangelical Christians.
"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," Romney said.
Romney said, "I disagree with Senator McCain on a number of issues, as you know."
The McCain campaign had said on Wednesday that McCain's lead was such that it was "virtually impossible" for either Romney or Huckabee to catch him.
(To read more about the U.S. political campaign, visit Reuters "Tales from the Trail: 2008" online at http://blogs.reuters.com/trail08/)