WASHINGTON — Senator McCain effectively sealed the Republican presidential nomination today as chief rival Mitt Romney suspended his faltering campaign. “I must now stand aside, for our party and our country,” Mr. Romney told conservatives.

“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 Senator Clinton or Obama would win. And in this time of war, I simply cannot let my campaign, be a part of aiding a surrender to terror,” Mr. Romney told the Conservative Political Action Conference in Washington.

Mr. Romney’s decision leaves Mr. McCain as the top man standing in the Republican race, with Michael Huckabee and Rep. Ron Paul of Texas far behind in the delegate hunt. It was a remarkable turnaround for Mr. McCain, who some seven months ago was barely viable, out of cash and losing staff. The four-term Arizona senator, denied his party’s nomination in 2000, was poised to succeed George W. Bush as the Republican standard-bearer.

Commenting on his front-runner status — a title he had and lost last year — Mr. McCain told the conference, “This time I now have that distinction and I prefer to hold onto it for quite a while.”

Messrs. McCain and Romney spoke by phone after Mr. Romney’s speech, though no endorsement was requested nor offered, according to a Republican official with knowledge of the conversation.

Mr. McCain prevailed in most of the Super Tuesday states, moving closer to the 1,191 delegates needed to win the nomination at this summer’s convention in St. Paul, Minn. Overall, Mr. McCain led with 707 delegates, to 294 for Mr. Romney, 195 for Mr. Huckabee, and Mr. Paul at 14.

Mr. Romney suspended his campaign, allowing him to hold onto his delegates. However, if Mr. McCain secures their support — combined with his own delegates — he would be nearly at the magic number and Mr. Huckabee would be mathematically eliminated. It is unlikely Mr. Romney would throw his support to Mr. Huckabee; the animosity between the two has pervaded the Republican race.

Mr. Romney launched his campaign almost a year ago in his native Michigan. The former Massachusetts governor and venture capitalist invested more than $40 million of his own money into the race, counted on early wins in Iowa and New Hampshire that never materialized and won just seven states on Super Tuesday, mostly small caucus states.

Mr. McCain took the big prizes of New York and California.

“This is not an easy decision for me. I hate to lose. My family, my friends and our supporters … many of you right here in this room … have given a great deal to get me where I have a shot at becoming president. If this were only about me, I would go on. But I entered this race because I love America,” Mr. Romney said.

There were shouts of astonishment, with some moans and others yelling, “No, No.”

Mr. Romney responded, “You guys are great.”

Mr. Romney claimed he was the true conservative in the race while Mr. McCain has been criticized by some on the right. Mr. McCain acknowledged the rocky relationship.

“I am acutely aware that I cannot succeed in that endeavor, nor can our party prevail over the challenge we will face from either Senator Clinton or Senator Obama, without the support of dedicated conservatives,” Mr. McCain said in prepared remarks to the same conference.

Mr. Romney acknowledged the obstacles to beating Mr. McCain.

“As of today, more than 4 million people have given me their vote for president, that’s of course, less than Senator McCain’s 4.7 million, but quite a statement nonetheless. Eleven states have given me their nod, compared to his 13. Of course, because size does matter, he’s doing quite a bit better with the number of delegates he’s got,” Mr. Romney said.

The Huckabee campaign said the former Arkansas governor would push on.

“We’re still in the race and we’re still competing for delegates, and today demonstrates how long and windy to the White House this is,” Huckabee’s campaign manager, Chip Saltsman, said.

Mr. Romney’s departure from the race came almost a year after his formal entrance, when the Michigan native declared his candidacy on February 12, 2007, at the Henry Ford Museum of Innovation in Dearborn, Mich.

Over the ensuing 12 months, Mr. Romney sought the support of conservatives with a family values campaign, emphasizing his opposition to abortion and gay marriage, as well as his support for tax cuts and health insurance that would benefit middle-class families.

“We need to teach our children that before they have babies, they get married,” he told voters at his campaign events.

But he was dogged by charges of flip-flopping, a criticism that undermined the candidacy of another Massachusetts hopeful — John Kerry in 2004. In seeking to unseat Senator Kennedy in 1994, Mr. Romney said he would be a better advocate for gay rights than his rival and he favored abortion rights.

Throughout his campaign, Mr. Romney was questioned by voters and the media about his Mormon faith. Hoping to assuage voters skeptical of electing a Mormon president, Mr. Romney spoke on December 6 in College Station, Texas, explicitly recalling remarks John F. Kennedy made in 1960 in an effort to quell anti-Catholic bias. He vowed to serve the interests of the nation, not the church, if elected president.

Fueled by what would grow to more than $40 million of personal donations, his campaign hired top-notch staff in the early voting states, and Mr. Romney scored an early win when his organization topped the field at the Iowa Straw Poll in August.

By that time, the national front-runners, Mr. McCain and Mayor Giuliani, had virtually ceded the lead-voting state to Mr. Romney.

Instead, Mr. McCain focused on New Hampshire, second on the calendar, while Mr. Giuliani employed an untested strategy of waiting out the early primary contests and instead staking his candidacy on a strong showing in the January 29 Florida primary.

Mr. Romney’s goal was to score back-to-back wins in Iowa and New Hampshire, clearing the field and creating momentum to roll through Florida — where he enjoyed the support of top aides to a former governor, Jeb Bush — and seal the nomination in the Super Tuesday contests.

Instead, Mr. Romney was beaten January 3 in Iowa by Mr. Huckabee, an ordained Baptist minister who received an unexpected outpouring of support in the caucuses from voters identifying themselves as evangelicals.

Five days later, Mr. Romney suffered a second consecutive defeat in New Hampshire, when Mr. McCain won the primary in part with the support of independents attracted to his self-styled maverick campaign.

Messrs. Romney and McCain went head-to-head in the January 15 Michigan primary, and Mr. Romney won, in part by highlighting his background as a business consultant and venture capitalist. When Mr. McCain acknowledged what seemed to be obvious, that not all of Detroit’s lost auto industry jobs would be recovered, Mr. Romney pounced.

As the calendar progressed, however, Mr. McCain picked up a big-ticket win in the January 19 South Carolina primary. Mr. Romney instead focused on his victory in the Nevada caucuses the same day.

Ten days later, the two squared off again in the Florida primary, where Mr. McCain scored a major upset after winning endorsements from the state’s two top elected Republicans — Governor Crist, a popular figure who had previously said he planned to remain neutral in the race, and Senator Martinez.

The following day, Mr. Giuliani dropped out of the race and endorsed Mr. McCain. A day later, Governor Schwarzeneger of California announced his endorsement of Mr. McCain, reflecting a coalescing of Republican support behind the senator as he approached a Super Tuesday showdown with Mr. Romney.