Mitt Romney's Republican rivals attacked his record as a private businessman and challenged him to release his federal income tax returns Monday night in the first of two debates before a pivotal weekend primary in South Carolina.

A Romney victory in next Saturday's balloting could seal his lock on the Republican nomination to challenge U.S. President Barack Obama in the November general election. The president — weighed down by a struggling American economy — is seen as vulnerable in his quest for a second White House term.

Relentlessly high unemployment, massive federal debt and millions of home mortgage foreclosures persist as the economy makes a laggardly recovery from the Great Recession of 2007-2009. Of all issues facing the United States, voters say repairing the economy is foremost.

Polls show Romney, the former Massachusetts governor, as the clear favourite in conservative South Carolina despite the mistrust or ambivalence of Republican voters who are unhappy with Romney's past stands on social issues like abortion.

Newt Gingrich and Rick Perry led the assault against Romney's record at Bain Capital, a venture capital firm that bought companies and sought to remake them into more competitive enterprises.

"There was a pattern in some companies ... of leaving them with enormous debt and then within a year or two or three having them go broke," Gingrich said. "I think that's something he ought to answer."

'I'm proud of my record'

Perry referred to a steel mill in South Carolina where, he said, "Bain swept in, they picked that company over and a lot of people lost jobs there."

"I'm proud of my record," the former Massachusetts governor responded, but he avoided an answer on making his tax returns public.

Romney said that the steel industry was battered by unfair competition from China. As for other firms, he said, "Four of the companies that we invested in ... ended up today having some 120,000 jobs.

"Some of the businesses we invested in were not successful and lost jobs," he said, but he offered no specifics.

It was Perry who challenged Romney, a multimillionaire, to release his income tax returns. The Texas governor said he has already done so, adding he believes Gingrich will do likewise later in the week.

"Mitt, we need for you to release your income tax so the people of this country can see how you made your money. ... We cannot fire our nominee in September. We need to know now."

Early this month, Romney won the leadoff Iowa caucus preference poll by just eight votes over Rick Santorum, the former Pennsylvania senator, but went on 10 days later to handily win the primary election in New Hampshire, where he maintains a vacation home.

Santorum, Gingrich, the former speaker of the House of Representatives, and Perry, the Texas governor, have all had a time in the limelight as the biggest threat to Romney, but all also have faded to also-ran status as Republicans appear to be coalescing around Romney.

Texas congressman Ron Paul, a strict libertarian who favours small government, has held steady through the first months of the campaign, but manages on average to poll only in the teens.

After the Monday debate, Republican candidates face a packed week of campaign events and one more nationally televised debate on Thursday. In the past three decades, no Republican has won the party's presidential nomination without carrying South Carolina.

Romney is hoping to overcome his history in South Carolina, where he struggled to a fourth-place finish during his 2008 White House run. The state has a large population of evangelicals and other conservative Christians, and concerns arose four years ago about his Mormon faith.

But Gingrich, Santorum and Perry all said Romney, after his victories in Iowa and New Hampshire, continued to benefit from the fractured Republican field and the failure of social conservatives to coalesce around a single alternative.

Jon Huntsman endorsees Romney

 The debate unfolded hours after moderate former Utah Gov. Jon Huntsman withdrew from the race and endorsed Romney.

That withdrawal raised the stakes of the debate — and one on Thursday night — for Gingrich, Santorum, Paul and Perry. Huntsman's withdrawal from the race, coincided with increasing pressure on Perry to leave the race to allow South Carolina's influential social conservatives to unify behind either former Santorum or Gingrich.

Santorum worked over the weekend at consolidating conservatives, trying to build support in South Carolina from the decision Saturday by an influential group of national Christian conservatives to back him.

Gingrich said he would "reassess" his candidacy if he lost in South Carolina and acknowledged that a Romney victory would mean "an enormous advantage going forward."

The field that remains after Saturday's vote will next compete in Florida on Jan. 31. The party does not officially name its candidate until the Republican National Convention in September.