PHILADELPHIA — The decision by Mitt Romney to suspend his presidential campaign all but ends the race in the Republican Party and makes John McCain its presumptive nominee.

Romney, 60, left the race for a number of reasons, one of which was to keep his options open for the future.

Had he refused to accept the seeming inevitability of McCain's nomination, he risked dimming his prospects for a 2012 candidacy, should McCain lose in the fall.

Romney spent more than $40 million of his own money on his campaign, as much as any major-party candidate has ever spent.

Thanks to his investment, he made himself a well-known figure across the country. He did not make himself the Republican nominee, at least not this year.

The former Massachusetts governor announced his departure Thursday during the annual Conservative Political Action Conference in Washington. There was some irony in the choice of venue, because Romney tried and failed to make himself the conservative standard-bearer.

'I have to now stand aside'

In his speech, Romney said that his staying in the race would serve only to prolong the Republican contest and make a Democratic victory in the fall more likely.

"If this were only about me, I would go on," he said, "but it's never been only about me. 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."

Romney, who did not endorse anyone Thursday, said that a Hillary Rodham Clinton or Barack Obama presidency was not acceptable. "I simply cannot let my campaign be a part of aiding a surrender to terror," he said.

McCain, speaking to the same group several hours later, congratulated Romney on running "an energetic and dedicated" campaign.

"We agreed to sit down together," McCain said, "and we agreed on the importance of uniting our party."

'This is a two-man race'

The other two candidates — former Arkansas Gov. Mike Huckabee and Texas Rep. Ron Paul — indicated that they'd stay in the race. However, they are far behind in the delegate count and have little chance of blocking McCain.

"This is a two-man race for the nomination, and I am committed to marching on," Huckabee said.

McCain has 707 delegates, more than half of the 1,191 needed to win. Romney has 294, Huckabee 195, Paul 14.

From the outset of the campaign, Romney, thanks to his fundraising prowess and his willingness to tap into his personal wealth, was a dominant presence.

He ran an early state strategy, hoping to win the Iowa caucuses and the New Hampshire primary. He led in those two states for most of 2007.

But Huckabee caught Romney in Iowa, largely on the support of evangelical Christians and other social conservatives. Then McCain caught him in New Hampshire.

Second-place finishes

Romney's second-place finishes in those two states were the unmaking of his strategy, although he gained new life by winning in Michigan, his birthplace, on Jan. 15.

Two weeks later, though, he lost in Florida, severely diminishing his chances.

He hung on through Super Tuesday. He was energized by polls showing him gaining in California, by conservative talk-show hosts who were desperate to stop McCain, and by the increasing focus of the electorate on economic issues, which he thought worked to his benefit.

As a highly successful businessman, Romney often said: "Sen. McCain says the economy is not his strong suit. Well, it is my strong suit."

On Super Tuesday, he won seven states, most of them small. He lost California, the one he needed.

At the conservative conference Thursday, Romney upstaged McCain, who had some fence-mending to do.

Over the years, the Arizona senator has broken ranks with conservatives on a number of issues, such as tax cuts and campaign-finance reform; many movement leaders distrust him.

'A few disagreements'

McCain, who received a reception that ranged from polite to enthusiastic, was jeered at one point when he talked about his position on immigration reform.

"We have had a few disagreements, and none of us will pretend that we won't continue to have a few," McCain told the crowd.

"But even in disagreement, especially in disagreement, I will seek the counsel of my fellow conservatives."