McCain, who defeated George W. Bush in the New Hampshire primary in 2000, has an edge among the state's independent voters.
And in a state that has a history of making or breaking presidential runs, candidates are pushing through exhaustion after Iowa, trying to woo the state's crucial independent voters with a message of "change."
The race has turned ugly between Romney and McCain, with the former Massachusetts governor trying to paint the Arizona senator as a Washington insider.
"I am responsible for the biggest change that has saved American lives," McCain argued Saturday at a campaign event, referring to his years-long call for the troop surge strategy in Iraq.
"It's one thing to say it; it's another thing to do it," Romney said Saturday in Derry, N.H., arguing his record in business and government proves he can overhaul a dysfunctional Washington.
Romney has also begun hammering the Arizona senator with a steady stream of negative ads in the state.
One such ad says, "He [McCain] even voted to allow illegal immigrants to collect Social Security."
But Romney went into the debate Saturday win a small victory under his belt after winning the Wyoming Republican caucus, giving him most of the 12 delegates at stake there.
Candidates debated Saturday night knowing that New Hampshire may reset a Republican presidential contest that has yet to find its front-runner.
A loss in the Granite State would further erode the candidacy of Romney, who has spent about $7.3 million on ads in New Hampshire, more than twice the $3.5 million spent by McCain, according to the Campaign Media Analysis Group.
During the first nine months of 2007, Romney dumped $17 million of his own fortune into his campaign. His campaign said this week that he contributed even more money during the fourth quarter.
While Saturday's ABC News debate didn't provide any clear winners, Romney's leading New Hampshire rivals demonstrated their willingness to go for his jugular in the hopes of wounding the once-frontrunner before the Jan. 19 South Carolina primary.
Post-debate, Romney's campaign were busily spinning the attention their candidate received.
"If they are ignoring you, they aren't worried about you," said Romney's spokesman Kevin Madden.
"He showed the greatest degree of poise on the stage," Madden added.
At a post debate party, Romney said, "I am happy to go to debates where all they talk about is me."
ABC News' John Berman contributed to this report.