It's a big day for politics in South Carolina Saturday on CNN. Tune in at 4 p.m. ET to see the candidates debate the issues from Thursday night's edge-of-your-seat debate then, at 6 ET, CNN's special live coverage of the South Carolina primary begins. See the results as they come in on CNN, CNN.com/Live and the CNN mobile apps.
(CNN) -- After two strong debate performances, momentum appeared to shift in Newt Gingrich's favor Friday as a key adviser predicted an almost-certain victory in Saturday's South Carolina primary and longtime front-runner Mitt Romney went on the defensive.
Romney, who enjoyed a 10-point lead over rival Newt Gingrich as recently as Tuesday, according to some polls, is now virtually tied for the lead in a four-candidate field that also includes former Pennsylvania Sen. Rick Santorum and U.S. Rep. Ron Paul of Texas.
In what appeared to be an effort to manage expectations ahead of the vote, a senior adviser to Romney said the longtime front-runner for the nomination is no cinch to win.
"Do I think we could lose South Carolina? Sure. Of course," strategist Stuart Stevens said after Thursday night's presidential debate in Charleston.
Richard Quinn, one of Gingrich's top advisers in the state, all but guaranteed a victory for Gingrich, saying that the former House speaker will walk away with "between a four and six point plurality win." Quinn is a longtime South Carolina GOP strategist who worked for former Utah Gov. Jon Huntsman, but signed on to advise Gingrich this week.
Rep. Tim Scott, an influential freshman congressman from South Carolina, remained undecided Friday on who he will vote for in Saturday's primary, though he acknowledged the recent gains made by Gingrich.
"I think the momentum has shifted," Scott said, crediting Gingrich's debate performances this week. "Challenge is you just don't want to pick a person because they had a good week."
For his part, Romney said Friday that he's "pretty confident" and "cautiously optimistic" about winning the critical state, where the winner of the GOP primary has gone on to win the nomination in each election since 1980.
"This is a campaign that's going to go the distance. I'm confident we're going to get the delegates we need," Romney told reporters in Gilbert, where he appeared with Gov. Nikki Haley. "And despite all the ups and downs in the campaign, in the final analysis, if I do my job right and get our supporters motivated, we'll be able to take the prize."
Romney said he knows South Carolina is an "uphill battle" for a candidate from Massachusetts, but he's "hoping and planning" to prevail. He touted his leadership skills, his "staying power" and "a message that connects."
"We're battling hard. The fact is that right now it looks like it's neck and neck. That's a pretty good spot to be in," he said, noting that he has "a lot of ground to make up" after coming in fourth in 2008.
The race has tightened as candidates crisscrossed the state. Unlike Iowa and New Hampshire, South Carolina is staunchly red. Saturday's results will reflect conservative sentiment and be a test for Romney, seen by some voters as more moderate than the others.
Four polls over the past two days show Romney's long-held lead in South Carolina nearly evaporating by the time the four took the debate stage, as Gingrich has surged into a virtual tie.
Gingrich on Thursday grabbed a major endorsement when Texas Gov. Rick Perry suspended his campaign and announced he was backing Gingrich.
But on Friday, Romney won the endorsement of a key Southern politician, Virginia Gov. Bob McDonnell.
"I'm a Southern governor endorsing Mitt Romney in the first Southern state primary," McDonnell said on CNN's "Starting Point with Soledad O'Brien." "The governor of South Carolina has endorsed him as well. I think hopefully that will help him some. He's the one that's been consistent. Other candidates have been up and down."
Romney's campaign said McDonnell would campaign with Romney in South Carolina on a busy Friday for the candidates.
The candidates made a flurry of public appearances after a boisterous nationally televised debate on Thursday.
Gingrich turned his contempt for the media into one of his strongest performances yet as he chastised the moderator, CNN's Chief National Correspondent John King, for starting off with a question about his marriage.
The question arose from interviews with ABC News and The Washington Post in which Marianne Gingrich, one of the candidate's ex-wives, alleged Gingrich asked for an "open marriage" before their divorce. Gingrich said the story was false. He married Bisek in 2000.
"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," Gingrich said.
Quinn, the Gingrich adviser, said he was still unsure how the moment was impacting the South Carolina race in its closing hours.
"It was kind of a big moment in the convention center," he said. "There was a sense in there that he knocked it out of the park in the first five minutes. But I don't know how it played for people in their living rooms."
Santorum's campaign got a boost when the Iowa GOP released the final certified results from its January 3 caucuses, which reversed Romney's eight-vote victory and instead determined that Santorum had won by 34 votes. He hammered that home on Friday in Lexington, poking Romney's "aura of inevitability" and his perceived moderation.
"He is no longer 2 and 0," Santorum said, referring to Iowa and Romney's New Hampshire win. "He is 1 and 1."
He added, "Conservative candidates in this race are going to pull a lot more votes than the moderate candidate in this race."
Santorum urged voters "to take a step back, get past the glib one-liners," and "get past the inevitability that the person with the most money wins."
When asked by CNN how well he has to do to have a viable path forward, Santorum was confident.
"I've won one of two primaries. This race just completely reshaped itself yesterday ... because I won Iowa, Rick Perry dropped out, Jon Huntsman dropped out -- I mean there are a lot of issues that came up at the last minute here that I'm not too sure will be reflected in Saturday's results," he said. "We feel like we are in a great position to be the conservative alternative and we're going to go out there and make that case."
Speaking at the Southern Republican Leadership Conference in Charleston, Paul reiterated his libertarian ideas.
"In many ways, we are murdering ourselves by our spending and our extension of troops around the world," he said.
CNN's Peter Hamby, Kevin Bohn and Dana Bash contributed to this report.