South Carolina voters head to the polls in their pivotal Republican presidential primary on Saturday, as surging Newt Gingrich eyed an upset win that could turn the race upside-down.

If the former House speaker defeats longtime frontrunner Mitt Romney, it could rekindle doubts about whether the more moderate former Massachusetts governor and multi-millionaire investor, champion of the party's establishment, can rally the party's conservative core, where he is viewed with suspicion.

And the champion in the first nominating contest in the US South will find the wind in his sails ahead of the January 31 primary in the vote-rich battleground of Florida.

The neck-and-neck rivals criss-crossed the state ahead of the primary, mindful that no Republican since 1980 has captured the party's nomination without first winning this southern bastion.

A victory here would be Gingrich's first triumph after Christian conservative former senator Rick Santorum squeaked out a victory in Iowa and Romney romped in New Hampshire.

"Tomorrow's going to be a very, very important day," Gingrich said at a packed campaign rally Friday in Orangeburg, adding he aimed to "win a shockingly big victory tomorrow."

"With your help and with the help of other good citizens across the state of South Carolina, we are going to take the first big step towards ensuring that a conservative is nominated for president of the United States," he said.

But Romney hoped a better-organized and richer campaign organization would ultimately carry him to victory here and put him on course to take on President Barack Obama in the November 6 elections.

"I think I said from the very beginning: South Carolina is an uphill battle (for) a guy from Massachusetts. I knew that. We're battling hard," he told reporters as his campaign abandoned once-confident predictions of victory.

A painful loss here, where he was once favored by nearly 20 points, would turn what Romney hoped would be a sprint to the nomination into a marathon where his rivals' momentum would face his more sophisticated, well-oiled operation.

Veteran Representative Ron Paul, a small-government champion opposed to foreign aid and overseas military intervention and backed by a devoted core of supporters, said he hoped the primary would "send a message to this country that we want less government. We want more freedom."

And Santorum told CNN his "huge upset" in Iowa showed the value of his take-no-prisoners conservative message and predicted he would "eventually end up with a one-on-one contest with Mitt Romney."

Romney has suffered a series of setbacks. Texas Governor Rick Perry dropped out of the race Thursday and endorsed Gingrich, and Iowa authorities rescinded Romney's eight-vote victory in Iowa's first-in-the-nation caucus, declaring Santorum the winner.

A Clemson University poll released on the eve of the primary here showed Gingrich with a six-point lead over Romney, 32 to 26 percent, with Paul in third with 11 percent and Santorum at nine percent.

And respected Washington website Real Clear Politics' average of recent South Carolina polling data showed Gingrich with a two percentage point lead over Romney.

Romney's once-substantial advantage at the national level may also be shrinking, with pollsters Gallup saying Friday that Romney's US-wide lead over Gingrich plummeted to 30-20 percent, compared with 37-14 percent a week ago.

Gingrich has risen with a series of feisty debate performances, and drew a standing ovation Thursday from the crowd with a blisteringly reply to a question about his marital woes, a query he denounced as "despicable."

Shirley Maddox, 72, who worked for a congressman when Gingrich was speaker and now works part-time for an insurance firm in Orangeburg, said she was leaning towards Gingrich.

"I've pretty much made up my mind but it could be changed -- probably Newt," she said, acknowledging misgivings about Gingrich's past infidelities but hoping "that's water under the bridge."

Amy Dent, 40, holding six-month-old daughter Philomena, the youngest of her eight children, said she was "still shopping, between Newt and Santorum. I'll wait until the final hours to make my final decision."

Romney meanwhile has sought to deflect attacks that he built his vast fortune while firing workers, saying he expected such jibes from Obama, not fellow Republicans -- traditionally the party of business.

And he still faces calls from his rivals to release his tax filings, in keeping with tradition.