MANCHESTER, N.H. — A former Massachusetts governor, Mitt Romney, clashed with Michael Huckabee on foreign policy and Senator McCain on immigration last night in a high-stakes presidential campaign debate three days before the New Hampshire primary.

“It’s not amnesty,” Mr. McCain shot back after Mr. Romney criticized his plan for overhauling the immigration system. “My friend, you can spend your whole fortune on these attack ads, but it still won’t be true.”

Earlier, Mr. Romney criticized Mr. 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 Mr. Huckabee, the former Arkansas governor.

“I read the article, the whole article,” Mr. Romney retorted.

Mr. Romney’s aggressive demeanor reflected the stakes in the wide-open race for the Republican presidential nomination. Mr. Huckabee defeated him in the Iowa caucuses on Thursday with an underfunded campaign. Now Mr. Romney faces a strong challenge from a resurgent Mr. McCain in New Hampshire’s first-in-the-nation primary next Tuesday.

A former senator, Fred Thompson, Mayor Giuliani and Rep. Ron Paul of Texas also shared the stage, but they were largely eclipsed for significant portions of the 90-minute debate as Messrs. Romney, McCain, and Huckabee struggled for advantage.

Mr. 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 Mr. McCain’s momentum had carried him into a narrow lead over Mr. Romney in New Hampshire, and that Mr. Huckabee was in third place. It also suggested Mr. 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 Messrs. Huckabee and McCain jabbed at Mr. Romney for having changed his position on numerous issues such as abortion, gun control, and gay rights.

“You are the candidate of change,” Mr. McCain said with a laugh.

And Mr. Huckabee, admonished not to characterize Mr. Romney’s position on the Iraq war, replied, “which one.”

Mr. Romney’s aides were at work challenging Mr. Huckabee’s truth-telling even when their candidate himself did not.

As the debate unfolded and Mr. Huckabee said he had supported President Bush’s decision a year ago to increase troop strength in Iraq, Mr. 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 Mr. McCain chatting with Senator Clinton.

Mr. McCain, whose candidacy appeared at the point of collapse last summer, sought to stress his national security credentials against major rivals whose political resumes are limited to governorships.

He said he had been the first one in the race to say the president’s initial strategy in the war in Iraq was not working, “And I again say that I’m glad to know that now everybody supported the surge.”

Mr. McCain added that “I was criticized by Republicans at that time. And that was a low point, but I stuck to it. I didn’t change. I didn’t say we needed a secret plan for withdrawal.”

Immigration emerged again in the debate as an issue that divided Messrs. McCain and Romney.

Mr. McCain has long backed a path to citizenship for millions of people living in the country illegally provided they meet certain requirements. Mr. Romney is running an ad that says Mr. McCain “wrote the amnesty bill that America rejected.”

“I’ve never supported amnesty,” Mr. McCain said, taking issue with the characterization and describing several steps immigrants must meet.

Mr. Romney allowed: “What he describes is technically true, which is his plan does not provide amnesty because he charges people $5,000 to be able to stay.”

All six men on stage sought to weave their way through a question about whether they would run on Mr. Bush’s foreign policy or run against it.

Mr. McCain said Mr. Bush deserves credit for his successes as he should take the blame for failures.

Mr. Huckabee said the administration’s arrogance was reflected in a former defense secretary, Donald Rumsfeld, saying an invasion force of 180,000 troops would be sufficient in Iraq.

Mr. Thompson agreed that the 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.”

Mr. Giuliani said Mr. Bush “got the big decision of his presidency right … when he put us on offense against Islamic terrorists.”

Mr. Paul, mounting a quixotic campaign, stuck to his insistence that the war should end.

Earlier yesterday, 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.

Mr. 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, Mr. McCain said of Mr. Romney: “We have significant differences but those differences will be aired in a positive and respectful manner.”

The debate was sponsored by ABC News and Facebook.