Newt Gingrich, on the verge of resurrecting his campaign, is aiming for his first primary win in Saturday's South Carolina contest as he puts Mitt Romney on the defensive over his tax records and fends off questions about his personal life.
The former House speaker enters the election with rising poll numbers and the publicity from two strong debate performances at his back.
Romney, who had been leading in South Carolina following his decisive win in the New Hampshire primary, acknowledged Friday that the race is now "neck and neck."
"I'm pretty confident, cautiously optimistic," Romney said, adding that his campaign will "go the distance."
But South Carolina, a state with a history of determining political fortunes, could be a turning point. Voters could reaffirm Romney as the race's front-runner, putting him in solid position going into Florida, or launch Gingrich into position as his top challenger and tee up a protracted battle for delegates.
Just one week ago, Romney seemed poised to go three-for-three in the first three contests, a historic feat. Then Gingrich's numbers started to surge following Monday's debate. Then the Iowa GOP announced that new certified results show Rick Santorum, not Romney, actually came out on top in that state's caucuses. Then Rick Perry bowed out, endorsing Gingrich.
Romney's campaign was handed a last-minute gift when ABC News announced it had an interview with Gingrich's second wife, Marianne, who claimed Gingrich once sought an "open marriage."
But Gingrich used the debate Thursday to shut the story down. He denied the claim, and to the audience's delight turned the tables on the moderator for asking him about the allegation.
"I am appalled that you would begin a presidential debate on a topic like that," Gingrich told the CNN moderator. "To take an ex-wife and make it, two days before the primary, a significant question in a presidential campaign, is as close to despicable as anything I can imagine."
He added, "I am frankly astounded that CNN would take trash like that and use it to open a presidential debate."
Gingrich's opponents mostly shied away from the topic, on the debate stage and the campaign trail.
Instead, Romney has tried to put pressure on Gingrich to release a more detailed accounting of the investigation into his ethical problems as House speaker, saying, "You know it's going to get out ahead of the general election."
"Given Speaker Gingrich's newfound interest in disclosure and transparency, and his concern about an 'October surprise,' he should authorize the release of the complete record of the ethics proceedings against him," Romney spokeswoman Gail Gitcho said in a statement.
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.
Gingrich, though, brushed off Romney's attempt -- an apparent bid to turn the tables on Gingrich for demanding Romney release his tax returns.
"I refuse to take seriously any request from the Romney campaign to disclose anything," Gingrich said. "Because ... they don't want to disclose anything at any level that involves them."
Romney faced boos from the audience Thursday when he reiterated that he doesn't plan on releasing his tax returns until April. He stood by the position again on Friday. He also pocketed the endorsement of Virginia Gov. Bob McDonnell.
Meanwhile, Santorum and Ron Paul, who came in second in New Hampshire, appear to be in a battle for third in South Carolina.
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.
The Associated Press contributed to this report.