CHARLESTON, S.C. - Front-runner Mitt Romney looked to fend off attacks from his rivals in a presidential candidates' forum Saturday a week ahead of the crucial South Carolina which may be their last chance to prevent him from becoming the Republican nominee to challenge President Barack Obama.

Five of the six candidates were scheduled to take part in the candidates' forum hosted by former Arkansas Gov. Mike Huckabee, an unsuccessful Republican presidential contender in 2008. Huckabee, an ordained Baptist minister, is popular among the state's many evangelical voters. Only Texas congressman Ron Paul is not participating.

Romney won the first two nominating contests, in Iowa and New Hampshire, and South Carolina, which holds the first primary in the South, may be his opponents' last chance to stop his march to the nomination. Although many South Carolina Republicans are unsure about Romney because of his moderate record as Massachusetts governor, he is benefiting from a divided opposition.

Newt Gingrich, former speaker of the House of Representatives; former Pennsylvania Sen. Rick Santorum and Texas Gov. Rick Perry are each making pitches for support among South Carolina's large bloc of socially conservative Republican voters.

Santorum got a boost Saturday when a group of about 150 social conservative leaders meeting in Texas voted to support him over Gingrich and Perry in a bid to united behind a single conservative alternative to halt Romney's momentum. But it was unclear whether the group's support will influence the South Carolina primary

The libertarian-leaning Paul, who finished in the top three in the first two nominating contests in Iowa and New Hampshire, made a brief stop in South Carolina on Wednesday before flying home to Texas. Early polls suggest Paul is in the top half of the field in South Carolina's Jan. 21 primary, but he's not expected back in the state until at least Sunday.

Romney defended his record in a new television advertisement that accuses Republican presidential rivals of taking the "Obama line" for criticizing him as a corporate raider during his time running the private equity firm Bain Capital.

Meanwhile, Romney's allies assailed Santorum in ads in South Carolina and Florida for advocating in the Senate for government spending on special projects in his home state of Pennsylvania. The goal was to keep Romney's challengers from gaining ground as Romney pushes for a three-state winning streak.

Romney's new ad lists Staples, Sports Authority and Steel Dynamics as successes of the Bain Capital venture firm.

"We expected the Obama administration to put free markets on trial ... Romney's (Republican Party) opponents are embarrassing themselves by taking the Obama line," the ad says.

That comment was a slap at Gingrich and Perry, who have been going after Romney in the days ahead of the South Carolina primary, over his Bain tenure and drawn criticism from across the Republican Party for doing so.

South Carolina may be fertile ground for the attacks on Romney. The state has suffered a string of shuttered textile plants and other workplaces. At 9.9 per cent, it has one of America's highest unemployment rates. And like its fellow Deep South states, its Republican electorate has a disproportionate number of blue-collar workers and noncollege graduates.

As if on cue, Obama's campaign released a scathing memo noting that Bain closed companies and cut wages and benefits, while Romney and his partners became wealthy. The memo amounts to a roadmap of the Obama campaign's general election playbook should Romney become the Republican nominee.

"His overwrought response to questions about it has been to insist that any criticism of his business record is an assault on free enterprise itself," Obama campaign aide Stephanie Cutter wrote. "But this is just an attempt to evade legitimate scrutiny of the record on which he says he's running."

Gingrich and Perry, each looking to right their struggling bids ahead of the primary, have described Romney as a greedy corporate raider, not the business-savvy job creator he professes to be.

They've been aided by a pro-Gingrich independent group that has pledged to run $3.4 million worth of ads attacking Romney on this issue in South Carolina. So far, less than $1.5 million in airtime has been bought for the ad, which features snippets of people talking about how they lost their jobs when Bain intervened at their companies.

Under pressure from conservatives to scale back the attacks on Romney's business record, Gingrich released a statement Friday asking the super political action committee to edit its advertisements to remove inaccuracies, or pull them.

But Gingrich pivoted to accuse Romney of failing as Massachusetts governor to post healthy job gains and argued that that record, too, would be fodder for Obama.

"These are just some of the facts which President Obama would use to undercut Gov. Romney's claims to be a job creator if he is the Republican nominee," Gingrich said.

Santorum has steered clear of the Bain fight as he aggressively competes in South Carolina, where polls show Romney with a narrow lead over Gingrich.

Some Republicans think Santorum is well positioned to rise in South Carolina, as he did just before the Iowa caucuses before narrowly losing to Romney. Santorum fared more poorly in New Hampshire but South Carolina is more friendly terrain for the champion of culturally conservative issues. Romney must reassure Christian evangelical voters who might be wary about his Mormon faith.