Earlier, Romney criticized Huckabee for having written that the Bush administration was guilty of an "arrogant bunker mentality" on foreign policy.
"Did you read the article before you commented on it," asked Huckabee, the former Arkansas governor.
"I read the article, the whole article," retorted Romney.
Romney's aggressive demeanor reflected the stakes in the wide-open race for the Republican presidential nomination. Huckabee defeated him in the Iowa caucuses on Thursday with an underfunded campaign. Now Romney faces a strong challenge from a resurgent McCain in New Hampshire's first-in-the-nation primary next Tuesday.
Former Sen. Fred Thompson, former New York Mayor Rudy Giuliani and Texas Rep. Ron Paul also shared the stage, but they were largely eclipsed for significant portions of the 90-minute debate as Romney, McCain and Huckabee struggled for advantage.
Romney walked on stage with his first win under his belt, a triumph in the scarcely contested Wyoming caucuses. The former Massachusetts governor, seeking to become the first Mormon president, said the outcome was "just the beginning."
A pre-debate poll suggested McCain's momentum had carried him into a narrow lead over Romney in New Hampshire, and that Huckabee was in third place. It also suggested Huckabee had not yet profited from his victory in Iowa, but the results of an election in one state often take several days to show up in surveys in another state.
Both Huckabee and McCain jabbed at Romney for having changed his position on numerous issues such as abortion, gun control and gay rights.
"You are the candidate of change," McCain said with a laugh.
And Huckabee, admonished not to characterize Romney's position on the Iraq war, replied, "which one."
Romney's aides were at work challenging Huckabee's truth-telling even when their candidate himself did not.
As the debate unfolded and Huckabee said he had supported President Bush's decision a year ago to increase troop strength in Iraq, Romney's campaign quickly emailed reporters with a Huckabee quote to a different effect. "Well, I'm not sure that I support the troop surge, if that surge has to come from our Guard and Reserve troops, which have really been overly stretched," it said he told MSNBC last January.
The event was part of a rare debate doubleheader, Republicans first, Democrats second, in the same hall at Saint Anselm College. Intermission brought White House hopefuls from both parties onto the stage at the same time, an unusual occurrence that left McCain chatting with Democratic New York Sen. Hillary Rodham Clinton.
McCain, whose candidacy appe administration had gone to war in Iraq without enough troops. "Presidents are not perfect. Policies are not perfect," he said, although he added, "we are on our way toward prevailing there."
Giuliani said Bush "got the big decision of his presidency right ... when he put us on offense against Islamic terrorists."
Paul, mounting a quixotic campaign, stuck to his insistence that the war should end.
Earlier Saturday, all the candidates made the rounds of restaurants, community centers and schools, engaging in the type of face-to-face campaigning New Hampshire voters demand. For most, talk of religious beliefs and abortion that was prevalent in Iowa gave way to low-tax, smaller-government pitches finely tuned for a state whose voters tend to care more about economics than social issues.
Romney's event in Derry showcased his newly embraced theme - change. One banner read "Washington is broken" while another contained an 11-item "To Do" list beginning with, "Make America Safer," and ending with, "Put people ahead of selfish interest."
In Peterborough, after his 100th New Hampshire town hall meeting, McCain said of Romney: "We have significant differences but those differences will be aired in a positive and respectful manner."
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Associated Press writers Glen Johnson, Philip Elliott and Holly Ramer contributed to this report.
