COLUMBIA, S.C. — Republican candidate Newt Gingrich on Tuesday leveled a forceful new attack on Mitt Romney, calling the GOP front-runner’s former private equity firm “exploitive.”

Gingrich had attacked Bain Capital before, but his comments before a gathering of business leaders in Columbia contained some of his harshest rhetoric yet and came just four days before the South Carolina primary, a critical benchmark for the Gingrich campaign.

After lagging behind in the Iowa causes and the New Hampshire primary, Gingrich is hoping a strong showing in the nation’s first Southern primary will re-energize his White House bid.

And he’s holding little back as he goes for broke against Romney.

The former House speaker said that, in at least some instances, the Bain model has meant “leverage the game, borrow the money, leave the debt behind and walk off with all the profits.”

“Now, I’ll let you decide if that’s really good capitalism. I think it’s exploitive. I think it’s not defensible,” he said.

Gingrich continued that what Romney engaged in “is not venture capital.”

“Venture capital is when you go in and put in your capital and you stick it out,” he said.

Gingrich has faced rebuke in some quarters as attacking the GOP bedrock of free enterprise in his criticism of Romney and Bain. But he argued Tuesday that raising questions about Romney’s track record at Bain should not be confused with an attack on capitalism.

“I’m proud of real capitalists. I’m proud of guys who say to their workers I’m in it with you. If I lose money and you lose a job we lost together because we both tried,” he said.

Earlier in the day Gingrich said at a town hall meeting in West Columbia that a Muslim-American seeking office in the U.S. would have to publicly renounce Islamic law to receive his backing.

When asked if he could support a Muslim for office, the former House speaker replied that it “would depend entirely on whether the person would commit in public to give up sharia,” or Islamic law.

Gingrich said he is totally opposed to Islamic law and supports a federal law that would pre-empt it.

“A truly modern person who worshipped Allah would not be a threat,” he said.