CHARLESTON, S.C. - With a week left to halt Mitt Romney from sweeping to a third straight victory, his GOP rivals are struggling in South Carolina for a theme, momentum and most crucially, one strong challenger to consolidate conservatives' misgivings about the front-runner.

The dynamics that lifted Romney to wins in Iowa and New Hampshire seem to be working for him here, even though South Carolina is often described as too evangelical and culturally southern for his background.

In some ways, the former Massachusetts governor is lucky, benefitting from a fractured opposition that has divided the anti-Romney vote for months. In other ways he is benefiting from shrewd and well-organized supporters. He uses TV ads to shore up his weaknesses and to batter the rivals he sees as most threatening.

In Iowa, the target was former House speaker Newt Gingrich, who plummeted under the barrage. In South Carolina, it's former Sen. Rick Santorum, a longtime champion of home-schooling, anti-abortion efforts and other social conservative causes.

Santorum nearly won the Iowa caucus, and some consider him the best bet for unifying the anti-Romney vote.

But a private group that supports Romney is pounding Santorum in South Carolina with TV ads and mailings. So is Rep. Ron Paul, the libertarian-leaning candidate who helped attack Gingrich in Iowa.

Paul's ads are especially harsh. They vilify Santorum for pushing pork-barrel projects as a Pennsylvania senator, and they portray him as an insincere conservative.

A group of social conservative leaders meeting in Texas voted Saturday to recommend Santorum as the Romney alternative. But a portion of them preferred Gingrich, who denied Santorum a two-thirds majority on their first head-to-head ballot, said Tony Perkins, president of the Family Research Council.

Perkins said the group's actions did not constitute an endorsement, adding that some participants will remain Gingrich supporters. He declined to say how he voted.

"Santorum was the preferred candidate by a significant majority," former presidential candidate Gary Bauer told The Associated Press by telephone from Texas. "They were all looking for the best Reagan conservative," he said. "It came down to things like, who do you most trust."

The Texas vote is obviously good news for Santorum. But it's unclear how much impact it will have in South Carolina's primary on Saturday.

The state is known for campaign surprises, and there's still time for twists and turns. Undercurrents of anti-Romney sentiment, perhaps fueled by his Mormonism, could be stronger than they seem.

But on the surface, at least, Romney is well-positioned with a week to go. If he wins South Carolina, only a seismic change in the campaign will keep him from becoming the nominee.

The next primary, on Jan. 31, is in Florida, a sprawling and expensive state where Romney's superior money and organization could essentially put the matter to rest, kicking off the general election against President Barack Obama.

"Romney is in good shape now, but the race is tightening," said LaDonna Ryggs, Spartanburg County GOP chairwoman.

There is little evidence that a barrage of ads depicting Romney as a heartless corporate raider is having much effect. He is airing a counter-ad defending his record at Bain Capital, which sometimes created jobs, and sometimes reduced them, when it restructured dozens of companies in the 1980s and '90s.

"That's what his job was, and he did it well," said Carleen Coffey, 51, who defended Romney even as she attended an event for Texas Gov. Rick Perry in Charleston.

The anti-Romney ad, aired by a group supporting Gingrich, has generated much comment in political and media circles.

Many conservative leaders have condemned it, and Gingrich later back-pedaled, questioning the accuracy of the anti-Romney documentary film behind it.

For ordinary South Carolina Republicans, however, the ad risks being lost in an avalanche of TV commercials, which many voters say they ignore.

Romney's campaign events run like clockwork, while his opponents often suffer glitches and modest crowds. Gingrich, in particular, has left people scratching their heads.

He spoke at a home-ownership rally Thursday in Columbia that appeared to be dominated by Democratic speakers and attendees. Gingrich got a big introduction at a GOP barbecue Friday in Duncan, but he inexplicably didn't show up for many minutes. Santorum jumped into the void, working the room and getting valuable one-on-one time with voters.

Then on Saturday, Gingrich's scheduled telephone conference with voters never took place. The dial-in number was invalid.

Perry has faded. Once seen having a good chance to beat Romney in South Carolina, the drawling Texan is drawing small crowds at cafes and restaurants. Saturday morning in Mount Pleasant, about half the people at Page's Okra Grill didn't bother to stop eating or talking while Perry spoke in a corner.

The TV attack ads in South Carolina skip Perry. It's a sign of his perceived insignificance, although he could benefit if the others slice each other up.

Former Utah Gov. Jon Huntsman is getting even less attention.

Some people think Santorum is rising, but the attack ads might slow him. Santorum's boyish looks have always boosted his image as a principled crusader for unborn children and other causes. But the ads being aired by Paul's campaign and the pro-Romney group depict him as a conniving, old-fashioned politician who grabbed federal money for his state whenever possible.

"Some people are going to be swayed," said Alexia Newman, a South Carolina GOP activist and Santorum supporter. "If you know about his records, you know the ads are false," she said. But that requires Santorum to break through the noise and clutter of political commercials flooding the airwaves.

The pro-Romney PAC, Restore Our Future, is running $1 million in ads in the state this week, and more than $800,000 next week. Not all of them target Santorum, however. Santorum's campaign and a PAC that backs him are running pro-Santorum ads.

No single issue is dominating the primary. That makes it harder for any one Romney opponent to catch fire.

Religion and the military play bigger roles here than in Iowa, and especially New Hampshire. Romney has worked hard to address both.

He has built several events around military service, starting with his Veterans' Day trip to South Carolina last November. He has been campaigning lately with Sen. John McCain, the 2008 presidential nominee and Vietnam War hero.

As for religion, Romney has tried to portray himself as a moral and faithful man, without going into details of Mormonism. On Friday, a woman in Hilton Head asked him, "Do you believe in the divine saving grace of Jesus Christ?"

"Yes, I do," Romney replied, adding: "Our nation was founded on the principle...of religious tolerance and liberty in this land, and so we welcome people of other faiths."

Romney's campaign has produced a Web ad in which an anti-abortion activist endorses him. Romney supported abortion rights as Massachusetts governor.