BOSTON — Former Massachusetts Gov. Mitt Romney yesterday made a Valentine's Day endorsement of Sen. John McCain, ending a bitter, yearlong rivalry and handing over almost enough delegates to guarantee McCain the Republican nomination.

Romney released the 280 delegates he had won from their pledge to support him and urged them to back McCain, calling him a "true American hero" and saying the party needs to unify behind him.

"Even when the contest was close and our disagreements were debated, the caliber of the man was apparent," Romney said at a press conference at his campaign headquarters here. "Right now the Democrats are fighting; let us come together and make progress while they are fighting."

That the two politicians eventually came together was not entirely surprising, as Romney is already looking to lay the groundwork for a future presidential run, and embracing the party's 2008 nominee could help that effort. McCain made a similar move in 2000, when he endorsed President Bush after a divisive primary fight. For McCain, the endorsement could help mend fences with conservatives in his own party, many of whom had rallied to Romney and view the senator warily.

But Romney's effusive praise for McCain was nonetheless jarring in light of his repeated criticisms, some as recently as two weeks ago, when both were in the final days of heated competitions for Florida and nearly two dozen Super Tuesday states.

The pair had clashed for more than a year as Romney spent millions from his personal fortune on television ads, many of which portrayed McCain negatively. The waning days of the campaign were especially nasty, with Romney accusing McCain of being dishonest and McCain attacking Romney as an inveterate flip-flopper.

McCain has accumulated 843 delegates, while former Arkansas Gov. Mike Huckabee has gathered 242 in his ongoing long-shot campaign. One of them must get to 1,191 delegates to become the nominee when the Republicans gather for their convention in Minneapolis.

Huckabee, campaigning in Wisconsin ahead of Tuesday's vote there, has vowed to compete until McCain acquires 1,191 delegates. He has dismissed speculation that a paid speech he will give to a youth leadership group in the Cayman Islands tomorrow is a sign he is slowing down his campaign, and aides said he had no plans to drop out.