Mitt Romney dispatched his surrogates Thursday to criticize Newt Gingrich's leadership ability and commitment to conservative principles, the start of an aggressive effort to derail his chief rival.

"He's not a reliable and trusted conservative leader," former Sen. Jim Talent of Missouri, a Romney backer, said during a conference call. Talent said Gingrich "says outrageous things that come from nowhere, and he has a tendency to say them at a time when they most undermine the conservative agenda."

Gingrich "is more concerned about Newt Gingrich than he is about conservative principle," said former New Hampshire Gov. John H. Sununu.

Gingrich brushed off the attacks during an appearance in South Carolina Thursday, insisting his campaign would not change its approach in response. "We're focused on remaining positive," he said.

As an example, Talent and Sununu held up a comment Gingrich made about Republican Rep. Paul Ryan's plan to balance the federal budget. The plan, popular among fiscal conservatives, would make significant changes to Medicare.

"I don't think right-wing social engineering is any more desirable than left-wing social engineering," Gingrich said of Ryan's plan during a May interview. "I don't think imposing radical change from the right or the left is a very good way for a free society to operate."

Romney's team pointed out Romney has offered his own plan for Medicare, instead of fully embracing Ryan's proposal. They held up Gingrich's comments as evidence that Gingrich isn't committed to the conservative cause because he undermined Ryan, a rising star in the party -- and they also said Gingrich doesn't have the restraint or temperament to be commander in chief.

Gingrich's comments on the Ryan plan represent "a reflection of the off-the-cuff thinking that he goes through to deal with issues, and that is not what you want in a commander in chief," said Sununu, who also served as White House chief of staff to former President George H.W. Bush.

The call, and the attack, is the first of several to come, Romney communications director Gail Gitcho said.

Romney's shift in strategy is significant: Romney's campaign has not previously criticized Gingrich head-on. It shows that with Iowa's caucuses drawing near, Romney views Gingrich as a real threat to his chances of winning the Republican nomination.

It also acknowledges that the nomination contest has become a two-man fight between Romney and Gingrich. Gingrich has risen to the top of polls both nationally and in critical early-voting states. In Iowa, a CBS-New York Times poll out Wednesday showed 31per cent of likely caucus goers support Gingrich, while 17 per cent back Romney.

It's part of a two-pronged strategy to attack Gingrich both personally and professionally.

While Talent declined to attack Gingrich's personal life -- Gingrich has been married three times and has acknowledged an extramarital affair -- the Romney campaign offers Romney's 42-year marriage to wife Ann as a contrast. A new ad highlights the marriage by showing old footage of Romney with his family and calling him "a man of steadiness and constancy."

Asked in South Carolina whether he believes Romney's ad represented an attack on Gingrich's family and is inappropriate, Gingrich said no. "They're fine," he said.

And during an appearance in Iowa Wednesday night, New Jersey Gov. Chris Christie, another supporter, focused on Romney's credentials as a family man. "This is a guy who is a father and a husband and loves his wife and his kids," Christie said.

Christie also implied that Romney's rivals could embarrass the country with their personal behavior. "When you look at candidates, say, `Is this the kind of person who's always going to make me proud in the Oval Office and never have to worry will

embarrass America? That I'll never have to worry will do something that will just make me ashamed?' (Romney) just won't," Christie said.