"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 one-time rival at his now-defunct campaign's headquarters. "This is a man capable of leading our country at a dangerous hour."
"Primaries are tough," said McCain, referring to their earlier rancor. "We know it was a hard campaign and now we move forward, we move forward together for the good of our party and the nation."
The two met privately before appearing together at a news conference. McCain had campaigned in Vermont and Rhode Island but added a flight to Boston to accept the endorsement.
McCain effectively sealed the nomination last week when Romney withdrew from the race; only former Arkansas Gov. Mike Huckabee and libertarian-leaning Texas Rep. Ron Paul remain. But neither has a chance to catch McCain in the convention delegate hunt.
In early primaries and caucuses, Romney collected 280 delegates. The number is enough to move McCain close to the total of 1,191 needed to clinch the nomination a full nine months before the November general election.
Huckabee was not ready to bow out.
"Right now there's a great big 'me, too' crowd coming together (for McCain)," Huckabee said in LaCrosse, Wis. "There's a lot of folks, sort of, in the establishment of the party that is not now wanting to be left out." He added, "I'm just not willing to be part of the coronation yet."
Romney's nod of support capped a bitter yearlong rivalry between the two men over the party's nomination. Romney criticized 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, Romney cast McCain as outside of the GOP's conservative mainstream and a Washington insider who contributed to the problems plaguing a broken system. McCain, in turn, argued that 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 Feb. 5, when McCain won a string of big-state primaries from coast to coast.
Officials said the former Massachusetts governor made his decision to back 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.
McCain is on a steady march toward amassing the 1,191 delegates he needs, but Huckabee has proven an unexpectedly durable challenger. With a strong appeal to evangelical conservatives, Huckabee defeated 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.
After Romney's announcement, McCain picked up endorsements from eight members of the Republican National Committee who will attend the convention and can support whomever they choose. That gave McCain 851 delegates, to 242 for Huckabee. It left Romney with 277 delegates.
While Romney can ask his delegates to support McCain, he won't be able to simply hand over all of them. 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.
Romney had support from six RNC delegates who continued to endorse him even after he dropped out of the race. Three of them switched to McCain, but some weren't ready.
"I will support our nominee," RNC member Diane Adams of Indiana said simply.
Other Romney supporters like Stewart Iverson in Iowa said they will work to rally others behind 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," Iverson said Wednesday.
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 GOP delegates.
Romney suspended his candidacy last week after it became apparent that toppling McCain would be near impossible to gain the delegates needed to defeat McCain.
He was the only one of McCain's main primary opponents who had resisted lining up behind the nominee in waiting; Rudy Giuliani and Fred Thompson both have endorsed him.
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Associated Press writers Mike Glover in Iowa, Todd Richmond in
LaCrosse, Wis., and Deanna Martin in Indiana contributed to this
report.
