Happy Valentine's Day, John McCain.

The GOP front-runner got a big political hug Thursday from former rival Mitt Romney, who in urging his own delegates to back the Arizona senator, put him in striking distance of sealing the nomination for good.

Romney and McCain were engaged in a bitter fight before the former Massachusetts governor suspended his race last week during an address to conservatives in Washington -- but at the announcement Thursday in Boston the two were like old friends.

"Things can get pretty rough in a political campaign, and in the thick of the fight it's easy to lose sight of your opponents' finer qualities. But the truth of the matter is that in the case of Senator McCain I could never quite do that," Romney said, highlighting McCain's dedication to national security. "Even when the contest was close and our disagreements were debated, the caliber of the man was apparent."

Romney suspended his campaign for the GOP nomination after McCain came out of the Super Tuesday contests with a huge lead in the delegate count -- but he did not offer an immediate endorsement.

Romney's backing Thursday could help McCain lock up the nomination faster than initially expected. Romney said he's encouraging his delegates -- which number 280 -- to back the Arizona senator, though they're not bound to do so.

But rival Mike Huckabee, who has 242 delegates, is vowing to stay in the race.

"I may get beat, but I'm not going to quit. It's just not in my nature," he told FOX News after the endorsement.

The latest delegate tallies show McCain has 843 delegates, a few hundred short of the 1,191 needed to secure the nomination. Sources tell FOX News that Romney decided to endorse earlier Thursday in the interest of helping McCain gain the delegates needed to seal the nomination and unite the party for the general election.

Romney cited similar reasons when he suspended his campaign last week, saying he didn't want to forestall the launch of a national campaign and hurt his party by staying in the race. Romney and McCain were in a tough race beforehand, with Romney persistently casting McCain as a Washington insider who is out of step with the GOP mainstream and McCain casting Romney as a flip-flopper.

McCain said Thursday that Romney's support is a "vital ingredient for victory in November," and that "We all know it was a hard campaign. Primaries are tough ... and now we move forward."

McCain already had the support of former New York mayor and GOP candidate Rudy Giuliani. But Romney's endorsement could help McCain in his quest to convince party conservatives that he shares their ideals, since Romney was the GOP pick of several prominent conservatives.

McCain's recent aura of inevitability has triggered the ire of conservative talk show hosts and pundits, but the Arizona senator has made headway in attracting the confidence of prominent elected conservatives, including House GOP Leader John Boehner and others in the Republican leadership.

Meanwhile, Huckabee's campaign argues there's no reason to drop out until somebody has 1,191 delegates.

Campaign manager Chip Saltsman told FOX News that it's "good news" Romney's delegates are being released.

"It means they are up for grabs," he said of the delegates, adding that Huckabee never expected to get Romney's endorsement.

While Romney can ask his delegates to support 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 Romney in the initial caucuses; the direction they go depends on whether they follow Romney's lead in endorsing McCain.

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

Six of 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.

FOX News' Shushannah Walshe and Serafin Gomez and The Associated Press contributed to this report.