Struggling to regain momentum after losing the Iowa caucuses, the former Massachusetts governor spent much of the night on defense, coming under attack by nearly every rival on stage, except for Ron Paul. (And he, too, probably would have dinged Romney if he’d gotten the time.) Romney was slapped by his opponents for changing positions on immigration, health care and foreign policy–though the jabs didn’t come unsolicited. Romney arrived on stage ready to fight, training his fire early on McCain and Mike Huckabee, who quickly fought back.

The fireworks started in the first minutes of the debate, which aired on ABC, when Romney trashed Huckabee’s recent essay in Foreign Affairs magazine in which the former Arkansas governor criticized the Bush administration for having an “arrogant bunker mentality” on foreign policy. In response, Huckabee pushed back, accusing Romney of echoing a similar opinion last year and of supporting a “timed withdrawal” of troops from Iraq.

“Don’t characterize my position,” Romney warned.

“Which one?” Huckabee shot back.

And that wasn’t the only venom of the night. When Romney went after McCain, accusing him of supporting amnesty for illegal immigrants (a charge Romney makes in one of his TV ads airing here in New Hampshire), the Arizona senator repeatedly denied the charge. “You can spend your whole fortune on these attack ads, my friend, but it still won’t be true,” McCain said, referring to the millions of his own dollars that Romney has pumped into his campaign.

McCain’s line elicited a loud cackle downstage from Rudy Giuliani, who has clashed more than once with Romney over immigration. Fred Thompson, a mostly quiet presence in the debate, went after Romney a few seconds later on the issue. “Didn’t you say Republicans were making a terrible mistake if they were separating themselves with President Bush on the illegal immigration issue?” Thompson asked. Romney said he’d been misquoted, which prompted McCain to jump back in. “You’re always misquoted,” McCain scoffed. “When you change positions on issues from time to time, you will get misquoted.” From downstage, Giuliani laughed again, and Romney was visibly irritated, accusing McCain of “personal attacks.”

But Romney didn’t get much of a reprieve. A few minutes later, Giuliani piled on Romney, reminding the audience that nobody in the field has a perfect record on immigration–not even Ronald Reagan, who Giuliani called “the hero of our party.” “Ronald Reagan did amnesty,” Giuliani said. “I think he’d be in one of Mitt’s commercials. This time, it was McCain who laughed.

While Huckabee, Thompson and Giuliani were clearly going after Romney out of their own self interest, it seemed at times as though they were doing McCain’s bidding. The latest CNN/WMUR/UNH poll out Saturday night has McCain narrowly leading Romney in New Hampshire 33 percent to 27 percent, with the rest of the pack down by double digits. That means every slam on Romney, no matter who delivered the punch, benefits McCain.

With some exceptions–notably his talk on health care, which is not his best subject–McCain brought his a-game to the stage tonight. The Arizona senator has struggled throughout the campaign to seem more energized and engaged in debate formats, which clearly aren’t his forte. But tonight, he broke out of the box, avoided repeating all-too familiar lines from his stump speech, as he has in the past, and was quick on his feet to engage Romney–though his last quip on Romney’s flip-flops (“You are the candidate of change.”) seemed a tad bit too pre-planned.

With the exceptions of their attacks on Mitt, everybody else faded to the background. Huckabee, who has made no secret of the fact he doesn’t expect to win in New Hampshire, did little to call attention to himself, which might have been calculated, since a win for McCain here would also be a big win for him. Ditto for Giuliani, who is still apparently focused on winning the nomination through the Super Tuesday states on Feb. 5. Thompson, as he has been for most debates, was slow and steady. His answers were good on many questions, but he still didn’t do much to help his cause.

Under attack from all sides, Romney wore down under the fire and appeared uneven and somewhat unfocused under the questioning. He never got the chance he needed to score points against McCain, his chief rival on Tuesday. Maybe he’ll have the fire and fortitude he needs by Sunday night, when the Republicans debate yet again, on the same stage, this time on Fox News Channel.