GILBERT, S.C. — Campaigning in the final hours before the South Carolina primary, Mitt Romney on Friday sought to turn a rival’s words against him, demanding that Newt Gingrich release documents from a decades-old ethics investigation.
Mr. Gingrich has been insisting that Mr. Romney release years of tax returns, saying that voters need to know such details before Republicans settle on a nominee.
With the sparring between the two at a fevered pitch as voting nears, Mr. Romney, at a stop in Lexington County, used a similar warning about Mr. Gingrich’s past, arguing that unreleased documents from the ethics investigation when Mr. Gingrich was in Congress in the 1990s could be used by Democrats if Mr. Gingrich becomes the nominee.
“He was pushed out of the House by his fellow members,” Mr. Romney said. “I think over 80 percent of Republican congressmen voted to reprimand the speaker of the House for the first time in history.”
Mr. Gingrich’s campaign responded quickly. R. C. Hammond, a spokesman for the campaign, used Twitter to send a link to a House Ethics Committee Web page with the public report of the case, and called the inquiry the product of “Nancy Pelosi’s ethics witch hunt.”
The back and forth reflected the race’s growing intensity in the state, with Mr. Gingrich claiming surging momentum and Mr. Romney’s campaign lashing out at Mr. Gingrich in every way it could.
Mr. Romney’s key supporters used the toughest language yet to describe Mr. Gingrich. John Sununu, the former governor of New Hampshire, called him a “megalomaniac, whose own leadership kicked him out as speaker because they got tired of megalomania.”
Mr. Romney conceded that new polls showed that his standing versus Mr. Gingrich appeared to be slipping, saying that he was “going to be tested here.”
“I said from the very beginning that South Carolina is an uphill battle for a guy from Massachusetts. I knew that,” Mr. Romney told reporters Friday. “We’re battling hard. Right now, it looks like it’s neck-and-neck. That’s a pretty good spot to be in.”
Mr. Romney, who has spent days talking about the release of his own tax returns, said Friday that he would release multiple years of information in April, rather than only the one year that he pledged to do earlier in the week.
“It will be more than one year, I just don’t know the exact number,” Mr. Romney said. “But we’ll have more than one year and people can take a look.”
The day actually began with a whimper when Mr. Gingrich canceled his first morning appearance because almost no one showed up.
With just hours left until the first primary in the South, voters in the state were being barraged by television commercials, fliers in their mailboxes and repeated phone calls from urgent-sounding campaign workers hoping to find and turn out supporters.
The debate could also be heard playing out across talk radio Friday morning as callers weighed in on the mounting attacks from the campaigns after a testy debate Thursday night.
Mr. Gingrich, the subject of intense attention as polls showed his support growing, had been scheduled to appear at the opening session of the Southern Republican Leadership Conference at 9 a.m., the start of a busy day in which the four candidates were set to make 19 appearances across South Carolina.
But on a day that he might have expected to address large, pumped-up crowds, Mr. Gingrich’s aides arrived to find the cavernous sports arena almost empty. Even as he was scheduled to start speaking, there were about 30 attendees in the arena and twice as many reporters.
A press aide to Mr. Gingrich then advised reporters to head to his next event, a tour of a nearby children’s hospital.
Bob Livingston, a former House colleague who is backing Mr. Gingrich’s presidential bid, took the stage at the event and announced that Mr. Gingrich had canceled because of “scheduling conflicts.” That prompted “boos” from the scattering of people in the arena.
But aides to Mr. Gingrich noted that the event was not organized by his campaign as a rally. Rather, it was supposed to be a group of activists who had gathered for their annual conference to discuss Republican politics.
The cancelation of Mr. Gingrich’s speech made for a slow start after a whirlwind 24 hours on Thursday that included the withdrawal of Gov. Rick Perry of Texas from the race and was capped off by a debate in which Mr. Gingrich angrily denounced the news media for reporting accusations by his second wife that he had asked for an “open marriage” to continue an affair with the woman who is now his wife.
Mr. Gingrich forcefully rejected the charges in Thursday’s debate, calling them “false” and chiding the news media for pursuing the story. The performance generated him a standing ovation.
Mr. Gingrich’s efforts to win in South Carolina appear to be bearing some fruit. Polling suggested on Thursday that he was gaining on Mr. Romney, who acknowledged as much in an early morning interview on Fox News.
“There’s a long road ahead,” Mr. Romney said on the “Fox and Friends” show. “Newt Gingrich is obviously a feisty competitor. We’ll have a contest that will take us, I’m sure, down the road a ways.”
Mr. Romney sought to return to his campaign’s main message on Friday with an open letter attacking Mr. Obama that appeared in several newspapers in South Carolina.
“ ‘Hope’ and ‘change’ were the watchwords we heard repeatedly from Barack Obama back when he was a presidential candidate campaigning here in South Carolina,” he wrote. “The change has come in the size and shape and reach of Washington. Barack Obama promised to fix our broken system. Instead, he’s grown it massively.”
Mr. Romney’s campaign has been distracted from that message for much of the last two weeks as he has tried to deflect attacks on his time at Bain Capital, a private equity firm, and questions about how much he pays in taxes.
Mr. Romney also sought to get back on message by announcing that he had received the endorsement of Gov. Bob McDonnell of Virginia. The two were scheduled to appear together at several events in South Carolina on Friday.
The other two remaining candidates in the race, Rick Santorum and Representative Ron Paul of Texas, were also scheduled to hold several events in the state on Friday.
Mr. Santorum aggressively went after Mr. Gingrich and Mr. Romney during the debate on Thursday night, saying that neither would offer the kind of contrast required to defeat Mr. Obama in the fall.
Speaking to the Southern Republican Leadership Conference gathering on Thursday before the debate, Mr. Santorum mocked his two leading rivals.
“How are we going to differentiate ourselves on the major issues of the day if we nominate Tweedledum and Tweedledee,” Mr. Santorum said, “instead of someone who stood up and said, ‘No’?”
The most contentious moments of the debate on Thursday night became fodder for conservative talk radio on Friday morning, with Republican voters weighing in with their judgments of the candidates.
Callers to WTMA Radio, the Charleston AM station that is the conservative megaphone of the Low Country, illustrated how divided much of the electorate is.
“I don’t care what Newt does in his bedroom,” a woman said, proclaiming that she was supporting Mr. Gingrich because she believed he would be the strongest debating opponent for Mr. Obama.
Jeff Zeleny reported from Gilbert, S.C., and Michael D. Shear from Washington. Jim Rutenberg, Trip Gabriel and