Spartanburg Herald-Journal, Michael Justus, Associated Press
Republican presidential candidate Mitt Romney meets supporters at Cherokee Trike and More in Greer, S.C., Thursday, Jan. 12, 2012.

COLUMBIA, S.C. — Republican presidential front-runner Mitt Romney cast himself Friday as a job-creating conservative in a new TV commercial that defends his time at the helm of a venture capital firm, private business experience under attack by his GOP rivals and, now, also by President Barack Obama's re-election campaign.

At the same time, Romney's allies were assailing rival and former Sen. Rick Santorum in ads in South Carolina and Florida for pork-barrel spending as they worked to keep the challenger, who has avoided criticizing Romney's business past, from catching fire while Romney pushes for a four-state win streak.

"Mitt Romney helped create and ran a company that invested in struggling businesses, grew new ones and rebuilt old ones, creating thousands of jobs," says Romney's new ad in South Carolina that lists Staples, The 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 GOP opponents are embarrassing themselves by taking the Obama line."

That line was a slap at Newt Gingrich and Rick Perry, who have gone after Romney over his Bain tenure and drawn criticism from across the GOP for doing so.

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 win the GOP nomination.

"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," top Obama aide Stephanie Cutter wrote. "But this is just an attempt to evade legitimate scrutiny of the record on which he says he's running."

Romney also rolled out a radio ad about values in South Carolina — likely to counter a TV ad by Gingrich that hits Romney on abortion — and one in Nevada promoting his jobs experience. Romney and his allies are the only presidential campaign entities on the air in Florida, running moderate to heavy levels of ads.

With the Bain issue now spreading across both the primary and general election campaigns, Romney was looking to blunt the force of attacks on the central rationale for his candidacy in hopes of preventing those criticisms from taking hold, if they haven't already. It's unclear whether attacks by Gingrich and his allies are having an impact on the race in South Carolina, where unemployment is high.

Gingrich and Perry, looking to right their struggling bids ahead of the state's Jan. 21 presidential primary, have described Romney as a greedy corporate raider, not the business-savvy job creator he has tried to portray himself as.

They've been aided by an outside group that supports Gingrich and 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.

Santorum, the former Pennsylvania senator, has steered clear of the Bain fight and is aggressively competing in South Carolina, where polls show Romney leading.

Some Republicans think Santorum is well positioned to rise in South Carolina, much like he did just before the Iowa caucuses, where he narrowly lost to Romney. Santorum fared more poorly in New Hampshire but South Carolina is more friendly terrain for the champion of culturally conservative issues.

The Romney-aligned super PAC called Restore Our Future, which is running $2.3 million in TV ads in South Carolina alone, is taking no chances of allowing a more conservative alternative to Romney to emerge and drag the race into a long slog through the spring nominating contests. The group, which spent nearly $3 million on advertising in Iowa attacking Gingrich, was seen as effectively ending Gingrich's rise in Iowa before the caucuses.

In addition to ads critical of Gingrich in South Carolina, the group run by former Romney aides is also now targeting Santorum on fiscal issues with TV ads and direct mail here and Florida, which holds the next primary on Jan. 31 and where Romney is considered the one to beat.

The commercials assail Santorum's support for pork-barrel — or earmark — spending while in Congress and votes to increase the federal debt limit. Both were common positions among Republicans, but have become flashpoints for conservatives angry about spending and the federal budget deficit.

"So how will Santorum beat Obama? Obama knows he can't," the ad says.

A flier filling mailboxes in South Carolina makes a similar pitch, stating: "America is in a financial mess because of politicians like Rick Santorum."

Santorum has stepped up his own advertising and a super PAC supporting him also is airing ads in South Carolina, although so far they have been upbeat messages about the candidate. The group confirmed Friday it was adding an additional $600,000 in advertising time in the state.