Republican presidential frontrunner Mitt Romney was conspicuous by his absence evening at a debate organized by an anti-abortion group in South Carolina.

Former Speaker of the House of Representatives Newt Gingrich, Texas Governor Rick Perry, ultraconservative former Senator Rick Santorum and Texas Representative Ron Paul all responded to the invitation to speak at Personhood USA's Presidential Pro-Life Forum ahead of Saturday's key primary vote.

All four of the candidates repeated their opposition to abortion, a subject on which Romney's position has varied over the years.

The president of Personhood USA, Keith Mason, said Romney was invited to the forum at a Greenville hotel in front of an audience of several hundred people.

"Why not Romney," Mason asked. "He was invited but he had a conflict and was not able to make it. He also had a conflict in Iowa."

At the next primary election in Florida, "we are holding two events," Mason said in a hint that he hoped Romney would attend.

Gingrich, the main challenger to Romney's lead in conservative South Carolina, said that while Romney was Massachusetts' governor, he supported a health care system he called "Romneycare" that provided "tax-paid abortions."

Gingrich argued the media and bureaucracy are "increasingly driving us towards a secular society that has nothing to do with US history and society."

He and the other presidential candidates at the forum hoping to take on President Barack Obama in a November election said they would challenge family planning funding, which some conservatives believe encourages abortion.

Paul, who spoke on a remote link from Washington, where he returned to vote against a bill to raise the nation's debt ceiling, said he spoke from experience as an obstetrician and gynecologist about the "obligation to protect all lives." By using ultrasound to show parents the foetus, he said he sometimes convinced women not to have abortions.

In South Carolina, where evangelical Christians make up about 60% of Republican voters, Romney has struggled to maintain his lead in public opinion polls.

With victories in Iowa and New Hampshire already in his pocket, Romney is hoping a win in South Carolina on Saturday will put him on an unstoppable path to being crowned the Republicans' presidential candidate.