BOSTON — A Republican campaign dropout, Mitt Romney, endorsed Senator McCain for the party’s presidential nomination and asked his national convention delegates to swing behind the likely nominee.

“Even when the contest was close and our disagreements were debated, the caliber of the man was apparent,” the former Massachusetts governor said, standing alongside his former rival at his now-defunct campaign’s headquarters. “As a party, we come together.”

Mr. McCain said: “We all know this was a hard campaign … and now we move forward, we move forward together for the good of our party and the nation.” Mr. McCain was campaigning in Vermont and Rhode Island earlier in the day and added a flight to Boston to accept the endorsement.

The two met privately before appearing together at a brief news conference.

Mr. McCain effectively sealed the nomination last week when Mr. Romney withdrew from the race; only a former Arkansas governor, Michael Huckabee, and Rep. Ron Paul of Texas remain. But neither has a chance to catch Mr. McCain in the convention delegate hunt.

In early primaries and caucuses, Mr. Romney collected 280 delegates. The number is enough to move Mr. McCain close to the total of 1,191 needed to clinch the nomination a full nine months before the November general election.

Mr. Romney’s nod of support capped a bitter yearlong rivalry between the two men over the party’s nomination. Mr. Romney criticized Mr. McCain in television ads in New Hampshire, and both candidates mixed it up almost daily during campaign events and debates. Neither is especially fond of the other.

Over the past year, Mr. Romney cast Mr. McCain as outside of the Republican conservative mainstream and a Washington insider who contributed to the problems plaguing a broken system. Mr. McCain, in turn, argued that Mr. Romney’s equivocations and reversals on several issues indicated a willingness to change his positions to fit his political goals.

The clash effectively ended on February 5, when Mr. McCain won a string of big-state primaries from coast to coast.

Officials said the former Massachusetts governor made his decision to back Mr. McCain earlier in the day, citing a desire to help the Arizona senator wrap up the nomination before too much more time passed and while Democrats still did not have a nominee.

Mr. McCain is on a steady march toward amassing the 1,191 delegates he needs, but Mr. Huckabee has proven an unexpectedly durable challenger. With a strong appeal to evangelical conservatives, Mr. Huckabee defeated Mr. McCain in two out of three states that chose delegates last weekend, and ran a far stronger race than expected before losing the Virginia primary on Tuesday.

The senator began the day with 843 delegates, to 242 for Mr. Huckabee.

While Mr. Romney can ask his delegates to support Mr. McCain, he won’t be able to simply hand over all 280 delegates. Many are from caucus states that won’t select the actual delegates until state conventions this spring. Those delegates will be selected by people who supported Mr. Romney in the initial caucuses; the direction they go depends on whether they follow Mr. Romney’s lead in endorsing Mr. McCain.

In other states, the delegates are bound to Mr. Romney, and their fate is governed by state party rules. In states like Montana, where Mr. Romney has 25 delegates, they would be free to support whomever they choose after Mr. Romney releases them.

Six of Mr. Romney’s delegates are members of the Republican National Committee who continued to endorse him even after he dropped out of the race. These RNC members are free to support any candidate they choose at the convention, and not all of them appeared eager to endorse Mr. McCain.

“I will support our nominee,” a RNC member, Diane Adams of Indiana, said.

Other Romney supporters like Stewart Iverson in Iowa said they will work to rally others behind Mr. McCain.

“My main focus is to try to bring Republicans together and say, he may not have been our choice in the caucuses but he is where we are today,” Mr. Iverson said yesterday.

In the next round of voting, Louisiana holds a state convention Saturday in which caucus-goers will help decide how 44 of the state’s 47 national convention delegates are split. At stake Tuesday in Wisconsin’s primary are 40 Republican delegates.

Mr. Romney suspended his candidacy last week after it became apparent that toppling Mr. McCain would be near impossible to gain the delegates needed to defeat Mr. McCain.