South Carolina primary: Gingrich surge troubles Romney
US presidential hopeful Newt Gingrich has surged in opinion polls ahead of voting in South Carolina, the latest leg of the battle to be the Republican challenger to Barack Obama.
Despite battling an ex-wife's claim that he wanted an open marriage, Mr Gingrich was 6% ahead of Mitt Romney in one eve-of-vote opinion poll.
Former Massachusetts Governor Romney said the race was "neck and neck".
Saturday's election is a crucial test for the four remaining contenders.
Polling stations opened at 07:00 local time (12:00GMT) and were due to remain open until 19:00.
Primaries and caucuses will be held in every US state over the next few months to pick a Republican nominee before the eventual winner is anointed at the party convention in August to take on Mr Obama in November.
Mr Romney swept into South Carolina more than a week ago buoyed by a double-digit opinion-poll lead, after his resounding victory in New Hampshire's primary and a near-tie in Iowa's caucuses.
But his aura of inevitability has since looked vulnerable as two muscular TV debating performances helped Mr Gingrich stage a fightback.
In a poll published on Friday from Clemson University, Mr Gingrich led in South Carolina on 32%, with Mr Romney on 26%, although 20% were undecided.
Texas Congressman Ron Paul was third on 11% and former Pennsylvania Senator Rick Santorum at 9%.
On Friday, Virginia Governor Bob McDonnell, the latest Republican to endorse Mr Romney, campaigned with him in South Carolina.
Mr Romney demanded that Mr Gingrich release documents relating to a past investigation against him for wrongdoing.
In 1997, Mr Gingrich became the first House of Representatives Speaker to be censured for ethics violations, and was fined $300,000 (£193,000).
Mr Gingrich, who has put Mr Romney on the defensive in recent days by urging him to release his tax returns, called his opponent's attempt to turn the tables a "panic attack".
The former House Speaker was buoyed by his own endorsement on Friday from action hero Chuck Norris, who called Mr Gingrich "the best man left on the battlefield".
During a forum on CNN on Thursday, Mr Gingrich skilfully turned a question about adultery into an attack on the media, drawing a standing ovation.
That debate was preceded by an interview his second wife gave to ABC News in which she claimed he asked her to share him with his mistress, then his congressional aide, now his third wife.
Mr Romney, on the other hand, was jeered during the same TV forum as he gave an evasive answer on how many of his tax returns he would disclose.
The private equity tycoon, who has an estimated wealth of up to $270m, said he would not release the forms until April, and declined to specify how many tax years would be disclosed.
Mr Romney has been on damage-limitation mode this week since revealing he pays a tax rate of about 15%, well below what most Americans pay, and after it emerged he has millions of dollars offshore in the Cayman Islands.
Some conservative Republicans view Mr Romney with suspicion because of his Mormon faith and political record as governor of liberal Massachusetts, but he is also admired for his unabashed championing of the free market.
He benefited in the first two nominating contests this month as the conservative vote fractured between Mr Gingrich, Mr Santorum and Rick Perry.
But Texas Governor Perry dropped out of the race on Thursday and endorsed Mr Gingrich.
Mr Santorum, who has trailed in the polls since his surprise photo finish with Mr Romney in Iowa's caucuses, made his final pitch to South Carolina's voters on Friday, labelling Mr Gingrich "radioactive".
But the Christian conservative, who lost his 2006 Senate race by 18 points, is trailing in polls and failing to draw big crowds at campaign events, despite endorsements from Christian evangelical leaders.
The Ron Paul campaign, meanwhile, is already looking ahead to contests beyond South Carolina.
The Texas congressman has a devoted following but some of his libertarian views, such as plans to axe defence spending, place him outside the Republican mainstream and he is viewed as a longshot candidate.
The economy is the top issue in this election, as President Obama seeks a second term amid voter frustration at the pace of recovery from the recession that began in 2007 during the tenure of George W Bush and ended in 2009.