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.

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.

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.

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

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.

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.

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.

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.

As for religion, Romney has tried to portray himself as a moral and faithful man, without going into details of Mormonism.

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

Romney’s main worries might involve currents he can’t see. South Carolina has a reputation for dirty campaign tricks, although many Republicans here say it’s mostly a thing of the past.

• Charles Babington & Thomas Beaumont, Associated Press