Associated Press

Republican presidential candidate and former Massachusetts governor Mitt Romney campaigns at American Legion Post 15 in Sumter, South Carolina.

Charleston - Mitt Romney's rivals contended on Sunday that his record at a private equity firm and as governor of Massachusetts would hobble him as the Republican nominee against President Barack Obama, hoping to slow the front-runner's momentum less than a week before the crucial South Carolina primary.

Former Pennsylvania senator Rick Santorum, Texas Governor Rick Perry and Newt Gingrich, the former speaker of the House of Representatives, all said Romney continued to benefit from the fractured Republican field and the failure of social conservatives to fully coalesce around a single alternative.

Gingrich acknowledged that if Romney won Saturday's primary, it would give him an “enormous advantage” going forward after back-to-back victories earlier this month in the first two contests in New Hampshire and Iowa. Gingrich said he would “certainly have to reassess” his own candidacy unless he captured South Carolina.

The state's senior US senator said a Romney victory probably would wrap up the nomination. “I think it should be over,” said Senator Lindsey Graham. “I'd hope the party would rally around him,” he told NBC's Meet The Press.

Romney took a rare day off from campaigning while his opponents focused on the South Carolina coast. They also attended church services and prayer breakfasts in a state with a large population of evangelicals and other conservative Christians.

The candidates faced a packed week of campaign events and nationally televised debates on Monday and Thursday before the first-in-the-South primary. No Republican has won the party's presidential nomination without carrying South Carolina, and polls show Romney leading in the state.

South Carolina is often described as too evangelical and culturally southern for Romney's background, but Romney and his supporters have been able to use TV ads to shore up his weaknesses and batter the rivals he sees as most threatening.

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

Santorum, who won the endorsement of an influential group of social conservatives and evangelical leaders on Saturday in Texas, said it was imperative for the field to shrink if conservatives had any chance of slowing Romney.

“We feel like once this field narrows and we get it down to a two-person race, we have an excellent opportunity to win this race,” Santorum told Fox News on Sunday.

Santorum battled Romney to a virtual tie in Iowa before falling to fifth place in New Hampshire.

Added Gingrich: “I think the only way that a Massachusetts moderate can get through South Carolina is if the vote is split.”

Gingrich, Perry and Santorum are continuing to compete for the support of social and religious conservatives. All three have the backing of well-financed independent groups known as super political action committee that can help keep their candidacies afloat. But a poor showing in South Carolina might push one or two of them out of the race.

Santorum said Republicans would have a hard time beating Obama in the November election if Romney were the nominee. Santorum cited Romney's push for mandatory health insurance coverage in Massachusetts.

“Romney's plan, as much as he'd like to say it's not, was the basis of Obamacare,” Santorum said on CBS' Face The Nation, referring to Obama's health care reform legislation which includes a mandate that people obtain medical insurance which Republicans view as unconstitutional.

“For us to give away that issue with Governor Romney would be a case, in my opinion, of malpractice on the part of the primary voters in the states to come.”

Gingrich and Perry used television interviews to focus on Romney's former leadership of the Bain Capital private equity firm. Many Republican and conservative leaders have rebuked his rivals for criticizing Romney's role at Bain. But both Gingrich and Perry defended raising questions about Bain's business practices, saying Romney's tenure would come under relentless assault from Democrats in the general election.

Romney's campaign claims he helped create more than 100 000 jobs while heading up Bain. But the Romney campaign cites success stories without laying out the other side - jobs lost at Bain-acquired or Bain-supported firms that closed, trimmed their workforce or shifted employment overseas.

Gingrich said questions about Bain were fair game since Romney has made his experience in the business world the chief selling point for his candidacy at a time when Obama is vulnerable due to high unemployment and the sluggish recovery from the Great Recession.

“It's fair to raise the questions now, get them out of the way now to make sure that whoever we nominate is clear enough, public enough, accountable enough that they can withstand the Obama onslaught,” Gingrich said.

Gingrich also said he planned to release his tax returns this week and called on Romney, who has refused to do so, to follow suit.

“He'll never get through the fall without releasing his records,” Gingrich said, insisting the country “deserves accountability and ... transparency”.

Perry suggested Obama's team was eager to attack Romney over his Bain tenure. That was a point Obama campaign adviser David Axelrod confirmed.

“If this is a fatal flaw we need to be talking about it now, not talking about it in September and October,” Perry said.

Axelrod portrayed a campaign against Romney as a debate over values and the needs of the middle class.

“Is that the economic vision for this country - outsourcing, off-shoring, stripping down companies, lowering wages, lowering benefits? I don't think that's the future for this country,” Axelrod told CNN.

Former Utah governor Jon Huntsman picked up the endorsement of The State, one of South Carolina's leading newspaper. Huntsman came in a weak third in New Hampshire after skipping Iowa, but the paper described him as a “realist” able to appeal to the centrist voters who will decide the general election.

Ron Paul was returning to campaigning for the first time since Wednesday. The libertarian-leaning congressman has spent several days at home in Texas after his second-place finish in the New Hampshire primary last week. - Sapa-AP