But on Thursday, a week after abandoning his quest for the White House, Romney made a switch that McCain could only welcome: He endorsed the Arizona senator.
"Things can get pretty rough in a political campaign, and in the thick of a fight, it's easy to lose sight of your opponent's finer qualities," Romney said during a tense appearance with McCain in Boston.
"This is a man capable of leading our country at a dangerous hour," he continued. "Sen. McCain understands the war we're in, the necessity of victory and the consequences of surrender."
The former Massachusetts governor yielded the lectern, then watched with a tight smile and clasped hands as McCain thanked him for running "a hard, intensive, fine, honorable campaign."
"Now we move forward together for the good of our party and our nation," McCain said.
Romney called on his delegates to the Republican National Convention to support McCain, which would put the senator close to the 1,191 that he needs to clinch the nomination. As of Thursday, McCain led the delegate count with 843, followed by Romney with 280 and former Arkansas Gov. Mike Huckabee with 242, according to the Associated Press.
A tangle of state laws and party rules leaves it unclear how many Romney delegates can easily switch to McCain.
"I really do think the vast, vast majority will vote for McCain now," said Dick Wadhams, the Colorado Republican chairman. "I think it more than locks up the nomination."
But with primaries ahead in Wisconsin, Texas, Ohio and other states, Huckabee vowed to keep campaigning until a candidate captures 1,191 delegates. "Right now there is a great big me-too crowd coming together," he said Thursday after a rally in La Crosse, Wis. It should come as no surprise, he said, that the GOP establishment is backing McCain.
"But what I'm concerned about is that there are a whole lot of people out here in the middle of America who are feeling left out," said Huckabee, who plans to take a weekend break from campaigning to deliver a paid speech in the Cayman Islands.
"They're feeling that their votes aren't even going to be counted, that nobody's paying attention to them. And that's why we need to keep the elections going."
Romney is the latest big-name Republican to endorse McCain since his Jan. 29 victory in the Florida primary made him the party's White House front-runner. Rudolph W. Giuliani, the former New York mayor who failed to win any states in his bid for the nomination, has also backed the senator.
Romney, who won 11 states, was McCain's harshest opponent, running TV attack ads against him in New Hampshire.
"Higher taxes, amnesty for illegals," an announcer said about McCain in one spot. "That's straight talk for being in Washington too long."
On Thursday, Beth Myers, who managed Romney's campaign, called her McCain counterpart, Rick Davis, and the pair made quick arrangements for a Boston detour on McCain's day trip to stops in Vermont and Rhode Island.
"I still have my views; the senator has his views," Romney said. "But as a party, we come together."
Finnegan reported from Washington and Rainey from La Crosse, Wis.