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The Republican and Democratic U.S. presidential candidates clashed on issues ranging from immigration to health care in back-to-back debates three days ahead of the first primary election in the White House race.

Watch FOX News Channel at 8 p.m. EST for a roundtable discussion with the Republican presidential candidates.

In the Democrats' debate Saturday night, Sen. Hillary Rodham Clinton, the front-runner no longer, accused campaign rival Sen. Barack Obama of changing his positions on health care and "a number of issues" in a debate leading up to the crucial New Hampshire primary on Tuesday.

"I have been entirely consistent in my position," countered Obama, adding that he and Clinton have a philosophical disagreement over her proposal to require Americans to purchase health insurance or face a penalty from the government.

Obama won the kickoff Iowa caucuses last Thursday, and his remaining rivals — Clinton, former Sen. John Edwards and New Mexico Gov. Bill Richardson — can ill afford for him to gain further momentum with a victory in New Hampshire's first-in-the-nation primary as well.

"You said you would vote against the Iraq war, you came to the Senate and voted for" funding, Clinton said, addressing Obama.

Obama's candidacy has soared on his pledge to bring change to Washington, and Clinton, the former first lady, sought to blunt his advantage.

Sunday morning, Clinton told supporters, "There's a big difference between talking and acting, between promising and performing. Over the next three days, I'm going to be making that case."

Edwards, the second-place finisher in Iowa, worked throughout the debate to align himself with Obama as an advocate for change in Washington, and described Clinton as a defender of the status quo.

Asked Sunday about an alliance with Obama, Edwards said, "I think there is a conviction alliance." Then he added, "First of all, I wouldn't go so far as to call it an alliance. Let me disagree with that. ..."

Whatever happens in New Hampshire and following states, he said on ABC's "This Week," "I'm in this through the convention and to the White House."

Obama and Clinton are locked in a dead heat in Hampshire, according to a poll released before the debate, each with 33 percent support. Edwards was in third place with 20 percent and Richardson got just 4 percent in the poll conducted by CNN and New Hampshire television station WMUR. The survey conducted Jan. 4-5 had a margin of error of 5 percentage points.

In the Republican debate, Mitt Romney displayed an aggressive demeanor, reflecting the stakes in the wideopen race for the Republican presidential nomination. Romney clashed with Mike Huckabee — who won the influential Iowa caucuses — on foreign policy and Sen. John McCain on immigration.

"It's not amnesty," McCain shot back after Romney criticized his plan for overhauling the immigration system in the debate Saturday night. "You can spend your whole fortune on these attacks ads, my friend, but it's not true."

On Sunday, Romney acknowledged: "I was incorrect."

The immigration argument was just one of several in which Romney was on the defensive.

"The guy with the ball is the guy people are trying to tackle," Romney told ABC News' "This Week" on Sunday. Pressed about the frequent characterization of him as a flip-flopper, Romney blamed his chief rival in New Hampshire, saying: "The McCain campaign from the very beginning did a masterful political job of trying to tag me as that."

Romney argued that his positions as he runs for president are consistent with the actions he took as governor — despite evidence that he has shifted to the right on some issues. He castigated politicians who he said are more interested in personal insults than changing government — even as his campaign sought to portray McCain as a nasty politician who has a record of personally attacking his opponents.

McCain, for his part, sought to walk a careful line.

"He has changed his position on almost every major issue," McCain said of Romney on NBC's "Meet the Press" on Sunday. He added: "That doesn't mean he's not a good person."

Earlier in Saturday's debate, 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," shot back Romney.

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 on Tuesday.

Mike Huckabee acknowledged "a brotherhood" of sorts with McCain, fueled by Romney's criticism. "We have both been brutally assaulted by Governor Romney with amazingly misleading ads that attacked and distorted and misrepresented our records, Romney attacking me in Iowa, attacking him in New Hampshire," Huckabee said on "Fox News Sunday."

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."

Two new polls in New Hampshire suggested McCain's momentum had carried him into a narrow lead over Romney, with Huckabee in third place. The surveys 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.

Saturday's event was part of a rare debate doubleheader, Republicans first, Democrats second, in the same hall at St. Anselm's College. Intermission brought White House hopefuls from both parties onto the stage at the same time, an unusual occurrence that left McCain chatting with Clinton.

There were a few moments of humor during the Democrats' debate.

"I've been in hostage negotiations that are a lot more civil than this," Richardson, a one-time diplomat, said at one point.

Asked what she could say to voters who do not find her likable enough, and seem to like Obama more, Clinton drew laughter. "Well, that hurts my feelings. ... But I'll try to go on."

She said she agreed that Obama was likable, then added, "I don't think I'm that bad."

That drew a wry response from Obama, who said, "You're likable enough, Hillary."