CONCORD, N.H. — As the Republican presidential candidates geared up Saturday for two crucial debates 12 hours apart, former House Speaker Newt Gingrich said the party’s core principles would be undermined if front-runner Mitt Romney wins the nomination.

“I think it would certainly lead to deep questions about what we stand for and who we are,” Mr. Gingrich told The Washington Times on Saturday. “I think it sends very conflicting messages. Clear Reagan conservatism is vastly more likely to put together a majority.”

With three full days of campaigning left before New Hampshire’s first-in-the-nation primary, Mr. Romney’s rivals intensified their efforts to emerge on Tuesday as the alternative to the former Massachusetts governor.

Mr. Romney, meanwhile, rallied his troops Saturday in a school gymnasium in Derry, where he trained his fire at President Obama and urged the crowd not to get overconfident with the wide lead in the polls.

He was joined by South Carolina Gov. Nikki Haley who told the crowd that a landslide victory here would give him an edge heading into her state’s Jan. 21 primary.

“This is history in the making. All eyes are on New Hampshire,” she said. “They are all watching you, and they are watching you to see how strong of a support you are going to send him to South Carolina with. You take care of him in New Hampshire, I’ll take care of him in South Carolina, and he’ll take care of us when he gets to The White House.”

Looking to slow Mr. Romney’s momentum, Mr. Gingrich confirmed that his campaign’s lawyers sent a letter Friday to television stations demanding that they stop airing an advertisement produced by a political action committee backing Mr. Romney. The ad funded by the Restore Our Future PAC suggests Mr. Gingrich was fined for ethics violations during his tenure as House speaker.

“These statements are NOT TRUE,” the letter states. “In fact, ANY statement, suggestion, or innuendo that Speaker Gingrich was assessed a congressional fine for violations of House Ethics Rules is fundamentally false and misleading.”

The letter asserts that Mr. Gingrich was asked to “reimburse the House $300,000 to offset some costs of the Committee’s investigation.”

The treasurer of Restore Our Future, Charlie Spies, stood by the ads and said the spots will air.

Mr. Gingrich said Saturday his legal team would consider litigation.

“Our lawyers think there may be some grounds to go after them,” he told reporters. “We wouldn’t rule out demanding that they [stations] take them down for being false.”

The candidates will debate at 9 p.m. Saturday at St. Anselm College in Goffstown, N.H., in a forum sponsored by ABC News/WMUR. They will face off again at 9 a.m. Sunday in Concord at a debate hosted by NBC’s “Meet the Press” and Facebook.

The debates are the first for the candidates since Dec. 15, when they were preparing for the Iowa caucuses.

Mr. Romney undoubtedly will be the target this weekend. Polls released Friday show Mr. Romney holding on to a commanding lead.

An NBC News/Marist poll Friday night showed Mr. Romney leading with 42 percent of likely Republican voters, followed by Texas Rep. Ron Paul at 22 percent and former Pennsylvania Sen. Rick Santorum at 13 percent. Mr. Gingrich and former Utah Gov. Jon Huntsman Jr. were tied at 9 percent, with Texas Gov. Rick Perry trailing the field at 1 percent.

Mr. Romney warned supporters the polls can be fickle.

“Those polls, they can just disappear overnight,” Mr. Romney said at a spaghetti dinner in Tilton, N.H., Friday night. “What you say to a pollster is a bit like going on a date. It’s like, ‘Well, I’m going to try this, but getting married, that’s something else.’ We need to make sure you’re working real hard.”

The NBC/Marist poll showed Mr. Gingrich’s support in New Hampshire plummeting and Mr. Santorum surging. In late November, the survey had Mr. Gingrich at 24 percent and Mr. Santorum at 2 percent. Since then, Mr. Gingrich endured a slew of negative ads in Iowa and Mr. Santorum fell only eight votes shy of defeating Mr. Romney there.

With the back-to-back debates looming, most of the candidates’ schedules for Saturday were relatively light. The exceptions were Mr. Huntsman, who is holding five events, and Mr. Santorum, who scheduled four campaign stops.

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