When McCain accused Romney of distorting his position, the former Massachusetts governor shot back: "Are they sent home? Are they sent home?"
"Some of them are, some of them are not, depending on their situation," McCain said.
The attacks turned personal when McCain delivered a sharp jab at Romney's latest strategy of attempting to position himself as the "change" candidate.
"I just want to say to Gov. Romney, we disagree on a lot of issues, but I agree you are the candidate of change," McCain said, a veiled swipe at Romney's new positions on on abortion and same-sex marriage.
Romney shot back: "The continued personal barbs are interesting but unnecessary."
Meanwhile, McCain tried to position himself as the candidate of experience, best able to face what he called "the transcendent challenge of the 21st Century, and that is radical Islamic extremists."
Turning toward McCain, Romney said that strategy wouldn't work if Sen. Barack Obama, D-Ill., was the Democratic nominee.
"If you think that making that argument as a Republican, that you have more experience, and have been around longer in the Senate, that you know the Senate cloakroom better than he does, that's not going to work," Romney said.
The debate came at a crucial time in New Hampshire, with McCain is looking strong in the state after his campaign all but collapsed this summer. Romney is working to win the support of Granite State voters after a disappointing finish in the Iowa caucus.
The latest WMUR/CNN poll released just hours before the debate began had McCain leading Romney in the Granite State by 6 percentage points, with the Arizona senator showing 33 percent support and the former governor 27 percent.
Former New York Mayor Rudy Giuliani garnered 14 percent. He has made frequent visits to the state and spent $2.5 million on television advertising there, according to data from the Campaign Media Analysis Group, an independent organization that tracks campaign spending.
Former Arkansas Gov. Huckabee, whose popularity among evangelical Christians helped him win the GOP caucus in Iowa, showed 11 percent support in New Hampshire, according to the WMUR poll.
The former Baptist minister had campaigned in New Hampshire, and his support for tax increases as Arkansas governor and his brand of economic populism isn't an easy sell with voters in the anti-tax, anti-big government state.
While Huckabee intends to campaign hard in New Hampshire until Tuesday, his next real test is the Jan. 19 South Carolina primary, where his prospects look better.
"We're going to have to go convert a lot of people in New Hampshire in the next five days. A big tent revival out on the grounds of the Concord state capital," Huckabee told ABC News senior political correspondent Jake Tapper before the debate.
Other Republican candidates have hardly registered a blip on the radars of voters in the state. The latest poll has Rep. Ron Paul, R-Texas, at 9 percent support, and former Sen. Fred Thompson of Tennessee and Rep. Duncan Hunter, R-Calif., tied with 1 percent support each.
Romney, the former governor of a neighboring state, has poured resources into New Hampshire, spending millions of dollars in the Granite State on television ads and building a campaign infrastructure. He has not detailed to reporters how much of his own fortune was spent there.