CHARLESTON, S.C. -- In an event aimed at swaying undecided South Carolina voters, five Republican presidential hopefuls showed up in Charleston to make the case for why they should be the next GOP nominee.

The forum, hosted by former Arkansas Gov. Mike Huckabee on Fox News, barred the candidates from attacking or even mentioning each other.

Newt Gingrich paid little attention to that rule. When asked to "defend the vilification of companies willing to put capital at risk in order to save failing companies," the former speaker zeroed in on Mitt Romney.

"Governor Romney ran saying he created 100,000 jobs in the private sector. ... I believe it's fair to ask that records be clear."

The hit on Romney backfired. Before Huckabee could finish trying to stop him, the audience drowned out Gingrich with boos.

The rest of the GOP field didn't mention their Republican rivals but were equally peppered with questions from the undecided voters.

For Romney, that meant defending against accusations he's a flip-flopper.

"I think you'll find that I've served as a conservative governor," Romney insisted. "I love this country, and I'm convinced that the principles of conservatism are what will keep America the shining city on a hill."

Rick Santorum, whom a growing chorus of Christian conservatives is now backing as the alternative to Romney, was asked how he could persuade liberal and independent voters should he win the nomination.

"Just go back and look at my record," Santorum said. "I've run five races, four of them I was victorious, all in heavily Democratic areas. I was able to go out and attract folks who didn't necessarily agree with me on [social] issues and frankly some other issues as well."

Jon Huntsman scored points for saying he'd wipe out the Department of Education, but it was Rick Perry who won the most laughs from the crowd.

When asked what kind of vice president he would pick, the Texas governor turned the question on his host.

"Mike, what you got going on next year?"

Ron Paul was the only candidate who did not participate.