By DAVID ESPO
AP Special Correspondent
CHARLESTON, S.C. - On the eve of a Southern showdown, Mitt Romney conceded Friday he's in a tight race with Newt Gingrich for Saturday's South Carolina primary in a Republican campaign suddenly turned turbulent.
It's "neck and neck," Romney declared, while a third presidential contender, former Sen. Rick Santorum, swiped at both men in hopes of springing yet another campaign surprise.
Several days after forecasting a Romney victory in his state, Sen. Jim DeMint said the campaign's first Southern primary was now a two-man race between the former Massachusetts governor, who has struggled in recent days with questions about his personal wealth and taxes, and Gingrich, the former House speaker who has been surging in polls after a pair of well-received debate performances.
The stakes were high as Republicans sought a challenger to Democratic President Barack Obama. Television advertising by the candidates and their supporters exceeded $10 million here, much of it spent in the past two weeks, and mailboxes were stuffed with campaign flyers.
In a bit of home-state boosterism, DeMint said the primary winner was "likely to be the next president of the United States."
Indeed, the winner of the state's primary has gone on to capture the Republican nomination each year since 1980.
A victory by Romney would place him in a commanding position heading into the Florida primary on Jan. 31. He and an organization supporting him are already airing television ads in that state, which is one of the country's costliest in which to campaign.
If the former Massachusetts governor stumbles in South Carolina, it could portend a long, drawn-out battle for the nomination stretching well into spring and further expose rifts inside the party between those who want a candidate who can defeat Obama more than anything else, and those whose strong preference is for a solid conservative.
Romney sounded anything but confident as he told reporters that in South Carolina, "I realize that I had a lot of ground to make up and Speaker Gingrich is from a neighboring state, well known, popular ... and frankly to be in a neck-and-neck race at this last moment is kind of exciting."
Left unspoken was that he swept into South Carolina 10 days ago on the strength of a strong victory in the New Hampshire primary and maintained a double-digit lead in the South Carolina polls for much of the week.
Campaigning in Gilbert, S.C., on Friday, Romney demanded that Gingrich release hundreds of supporting documents relating to an ethics committee investigation into his activities while he was speaker of the House in the mid-1990s.
""Of course he should," he told reporters. Referring to the House Democratic leader, he said, "Nancy Pelosi has the full record of that ethics investigation. You know it's going to get out ahead of the general election."
That was an attempt to turn the tables on Gingrich, who has demanded Romney release his income tax returns before the weekend primary so Republicans can know in advance if they contain anything that could compromise the party's chances against Obama this fall.
Gingrich's campaign brushed off Romney's demand, calling it a "panic attack" brought on by sinking poll numbers.
"Don't you love these guys?" the former speaker said in Orangeburg. "He doesn't release anything. He doesn't answer anything and he's even confused about whether he will ever release anything. And then they decide to pick a fight over releasing stuff?"
In January 1997, Gingrich became the first speaker ever reprimanded and fined for ethics violations, slapped with a $300,000 penalty. He said he'd failed to follow legal advice concerning the use of tax-exempt contributions to advance potentially partisan goals, but he was also cleared of numerous other allegations.
At the same time he fended off a demand on one front Friday, Gingrich was less than eager to face further questions made by his second wife, Marianne, who said in an ABC interview broadcast Thursday night that he had once sought an open marriage so he could keep the mistress who later became his current wife.
He denies the ex-wife's account.
On his final lap through the state, Santorum campaigned as the Goldilocks candidate - just right for the state's conservative voters.
"One candidate is too radioactive, a little too hot," he said, referring to Gingrich. "And we have another candidate who is just too darn cold, who doesn't have bold plans," he added, speaking of Romney.
His campaign also announced endorsements from conservative leaders in the upcounty portion of the state around Greenville, where the heaviest concentration of evangelical voters lives.
Santorum, a former Pennsylvania senator, dismissed Texas Rep. Ron Paul, the fourth contender in the race. "There are four, three of whom have a chance to win the nomination," he said, including himself.
Paul, who finished third in the Iowa caucuses and second in the New Hampshire primary, has had a limited presence in South Carolina.
But he flew to six cities on a burst of campaigning on the race's final day, and drew applause for having returned to Washington, D.C., earlier in the week to vote against Obama's requested increase in the debt limit.
"When you hear the word principle, you think of Ron Paul. He's the embodiment of that," said Derek Smith, a 26-year-old engineer for the Navy in Charleston. "If he were to run as a third-party candidate, I would vote for him unconditionally."
Paul has said he has no intention of doing that.
Interviewed on C-SPAN, Santorum said the race "has just transformed itself in the last 24 hours." It was hard for any of the campaigns to argue with that.
In a bewildering series of events on Thursday, Romney was stripped of his victory in the Jan. 3 Iowa caucuses by state party officials, who said a recount showed Santorum ahead by 34 votes.
Then came an unexpected withdrawal by Texas Gov. Rick Perry, who endorsed Gingrich. But Gingrich was suddenly caught in a controversy caused by his ex-wife's accusations.
At a two-hour debate that capped the day, Gingrich drew applause when he strongly attacked ABC and the "liberal news media" in general for injecting the issue into the final days of the South Carolina campaign.
By contrast, Romney faced a round of boos from the audience when he stuck by earlier statements that he would wait until April to release his tax returns.
Romney has stumbled several times in recent days, including once when he said he paid an effective tax rate of about 15 percent. That's half what many middle-income Americans pay, but it's what the law stipulates because his income derives from investments, which are taxed at a lower rate than wages.
Gingrich posted his own tax returns online during the Thursday debate, reporting he paid 31.5 percent of his income to the IRS.
Associated Press writers Charles Babington, Kasie Hunt, Thomas Beaumont, Philip Elliott, Beth Fouhy and Shannon McCaffrey contributed to this report.