WASHINGTON — Mitt Romney defended his record Friday 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 a private equity firm.

Meanwhile, Romney's allies assailed rival and former Sen. Rick Santorum in ads in South Carolina and Florida for government spending on localized projects while also working to keep challengers from gaining ground as Romney pushes for a four-state win streak.

Romney is campaigning in South Carolina, where voters are still unsure of him ahead of the state's crucial Jan. 21 primary. Romney won the first two nominating contests, in Iowa and New Hampshire, and South Carolina may be his opponents' last chance to stop his momentum and prevent him from becoming the Republican presidential nominee.

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 former speaker of the House of Representatives Newt Gingrich and Texas Gov. Rick 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 the nation'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.

Rick Santorum, the former Pennsylvania senator, has steered clear of the Bain fight as he aggressively competes in South Carolina, where polls show Romney leading.

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.