Richard Shiro, Associated Press
FILE -- In this May 5, 2011 file photo, South Carolina Governor Nikki Haley speaks to the South Carolina Greenville Tea Party's "1st Presidential Debate Freedom Rally" in Greenville, S.C. Thursday, May 5, 2011. Haley endorsed Mitt Romney Friday morning, saying Romney is "someone that knows what it's like to make a decision and lead."

SIOUX CITY, Iowa — Republican presidential hopeful Mitt Romney scooped up a coveted endorsement Friday from South Carolina's governor as he sought to halt the momentum of rival Newt Gingrich in a final weekend of campaigning for the GOP nomination before the holidays.

South Carolina Gov. Nikki Haley, a rising GOP star, said Romney is "someone that knows what it's like to make a decision and lead." And she said President Barack Obama seems most scared of facing Romney in the general election. She announced her endorsement Friday on Fox News.

Romney, campaigning in western Iowa for the Jan. 3 caucuses, planned a swing later Friday with Haley through South Carolina, the first Southern state to hold a primary election.

Gingrich was returning to Washington after defending his conservative credentials Thursday night in a nationally televised GOP debate.

Minnesota Rep. Michele Bachmann attacked Gingrich during the debate, saying he "had his hand out and received $1.6 million to influence senior Republicans and keep the scam going in Washington, D.C.," for Freddie Mac, a government-backed housing entity.

"Just not true," Gingrich shot back. "I never lobbied under any circumstances," he added, denying an allegation she had not made.

The clash underscored the state of the race, with Gingrich, the former House speaker, atop the polls nationally and in Iowa and Romney, Texas Rep. Ron Paul and his other pursuers working in television ads and elsewhere to overtake him.

Former Sen. Rick Santorum of Pennsylvania, who has staked his campaign on Iowa, was quick to challenge Gingrich as a conservative leader. He recalled that Gingrich had to contend with a "conservative revolution" from the ranks of Republican lawmakers when he was House speaker in the 1990s.

Romney, who runs second in polls in Iowa, largely refrained from criticism of Gingrich, despite increasingly barbed attacks in day-to-day campaigning. Instead, he firmly rejected suggestions that he had once favored gay marriage only to switch his position. "I have been a champion of protecting traditional marriage," he said.

Given the stakes, Gingrich, Bachmann and Santorum weren't the only contenders eager to impress Iowa voters and a nationwide television audience with their conservative grit.

"I hope I am the Tim Tebow of the Iowa caucuses," said Texas Gov. Rick Perry, referring to the Denver Broncos quarterback whose passing ability draws ridicule but who has led his team to a remarkable seven victories in eight weeks.

"We're getting screwed as Americans," said former Utah Gov. Jon Huntsman, insisting that he, in fact, was a steadier conservative than any of the others on stage.

"Anybody up here could beat Obama," said Paul, whose views verge on libertarianism and who has struggled to expand his appeal.

Bachmann, who was quicker than any other candidate to criticize her rivals, bristled when challenged repeatedly on the accuracy of her facts. "I am a serious candidate for president of the United States, and my facts are accurate," she said.

Indeed, the big question in the opening moments of a fast-paced, two-hour debate went to the heart of a dilemma that could eventually settle the race: Do conservative Republican caucus and primary voters pick a candidate with their hearts, or do they look elsewhere if they judge their favored candidate might not be able to defeat the president?

Those voters begin making that choice on Jan. 3, and if experience is any guide, one or more of the presidential hopefuls on the debate stage will not make it out of Iowa to compete in the New Hampshire primary a week later.

Gingrich, who seemed an also-ran in the earliest stages of the race, has emerged as a leader heading into the final stretch of the pre-primary campaign. His decades in Washington and his post-congressional career as a consultant have been the subjects of tough critiques from Romney's campaign in the past week.

But the former speaker passed up an offer to criticize his rival on the issue of Medicare, saying, "I'm not in the business of blaming Gov. Romney." In fact, he said, Romney has made constructive suggestions for preserving the program that tens of millions of Americans rely on for health care yet faces deep financial woes.

Gingrich drew criticism earlier in the year for calling a GOP Medicare proposal "right-wing engineering." Romney refrained from criticizing that plan but did not embrace it in full.

Bachmann, who has long-since faded to the back of the pack in the polls, showed no such reluctance.

When he labeled her charges inaccurate, she shot back that when she made similar contentions in the previous debate, she was judged factually accurate by an independent arbiter — a claim that the website Politifact deemed inaccurate. She said Gingrich's work for Freddie Mac was in furtherance of a "grandiose scam" to keep alive an entity at the heart of the housing crisis.

"I will state unequivocally for every person watching tonight: I have never once changed my positions because of any payment," Gingrich said, adding that, in fact, he favored breaking up both Fannie Mae and Freddie Mac, his benefactor.