Barack Obama's campaign has fired off a withering attack on Mitt Romney, branding the US president's likely election foe as a corporate raider who made money 'hand over fist' by destroying jobs.

The Obama campaign's intervention Friday whipped up a new storm around Romney, who has been battered by criticism from his Republican rivals over his 15-year role at equity firm Bain Capital and his claims to have created 100,000 jobs.

The Chicago-based team's decision to enter the fray added to the impression that an important moment of the 2012 campaign could be at hand, with Romney battling to shore up the central rationale of his presidential run.

Obama strategist Stephanie Cutter took aim in a memo at Romney's claim that his business expertise gives him the corporate savvy and turnaround skills needed to reboot the struggling US economy.

She accused Romney of taking advantage of an "uneven playing field" by using the cash of rich investors to take over failing firms, strip them down and fire workers during his time at the private equity firm.

"Our economic crisis and endemic income inequality were caused in large part by a few who put profits over people," Cutter wrote.

"Mitt Romney and his friends made money hand over fist while working families lost their grip on the middle-class lifestyle they earned.

"Between now and November the American people will decide whether to respond to this crisis by electing a corporate raider who profited from -- and promises to restore -- the conditions that caused it."

Cutter accused Romney of closing more than 1,000 industrial plants, stores and offices, cutting employee wages and benefits and pensions and outsourcing American jobs to other countries while making hundreds of millions of dollars.

The controversy was further fueled by a report by the McClatchy newspaper chain that Bain Capital more than doubled its money on its acquisition of GS Industries Inc., the former parent company of Georgetown Steel, in the 1990s, even as the steel manufacturer went on to cut more than 1,750 jobs.

Obama would restore fairness for consumers and help the middle classes regain economic security and support the business sector, Cutter argued, and rejected Romney's claims that Obama was waging war on capitalism.

"That's what's on trial, not free enterprise. Free enterprise isn't running for president, Mitt Romney is."

Romney sought to mitigate political damage from the Bain affair, which is complicating his campaign to win next week's South Carolina primary after triumphs in Iowa and New Hampshire nominating contests.

The South Carolina primary is seen as a last chance for more conservative candidates Newt Gingrich, former senator Rick Santorum and Texas Governor Rick Perry to slow Romney's march to the nomination.

Romney released a political ad branding himself as a 'conservative businessman' responsible for blue chip brands such as Sports Authority and Staples.

"Mitt Romney helped create and ran a company that invested in struggling businesses, grew new ones and rebuilt old ones, creating thousands of jobs -- those are the facts," the ad's narrator said.

"We expected the Obama administration to put free markets on trial, but as the Wall Street Journal said, 'Mr Romney's (Republican) opponents are embarrassing themselves by taking the Obama line.'"

Campaigning in South Carolina, Romney savaged Obama's record and accused him of resorting to class warfare with his campaign's attacks.

"They seem to want to replace ambition with envy," said Romney, who also got a boost from defeated 2008 Republican presidential candidate John McCain who was traveling with him.

McCain said that Romney's career at Bain was 'what the free enterprise system is all about and jobs and businesses were created all over this country by Mitt Romney'.

Democrats are trying to keep the Bain Capital controversy alive after a political action committee backing Gingrich released a movie featuring tearful vignettes of people who blame Bain Capital for culling their jobs.

Romney, after his twin nominating contest wins, is still the hot favorite for the Republican nomination and is seen by many analysts as the most formidable candidate to take on Obama in November's election.

An American Research Group poll among likely Republican voters in South Carolina Friday showed the former Massachusetts governor leading the field, with 29 per cent, ahead of Gingrich on 25 per cent and libertarian favorite Ron Paul on 20 per cent.