Former House speaker Newt Gingrich exhorted conservatives to rally behind his ascendant candidacy as he bid for an upset victory in Saturday’s South Carolina primary over former Massachusetts governor Mitt Romney, who only a week ago appeared to be on an easy march to the Republican presidential nomination.

With two strong debate performances this week and missteps over taxes by his rival, Gingrich has managed to puncture the aura of inevitability that surrounded Romney. A Gingrich victory Saturday would extend the race on to Florida, whose primary is Jan. 31, and possibly well beyond, with the party divided between its insurgent and establishment wings.

Appearing increasingly confident as he campaigned Friday, Gingrich said that with the consolidation of the conservative vote, he could win “a shockingly big victory” Saturday. “The only effective conservative vote to stop the Massachusetts moderate is to vote for me,” he told an overflow rally in Orangeburg. “That’s what all the polls are saying now.”

Romney, recognizing the trend lines, began to lower expectations about the state’s primary and its effect on the GOP race. “I said from the very beginning South Carolina is an uphill battle for a guy from Massachusetts,” Romney told reporters after a rain-soaked outdoor rally on a muddy farm in Gilbert.

Polls in South Carolina show a race that has changed dramatically in a matter of days. Gingrich has surged, and Romney has slumped. An NBC News/Marist poll released Thursday showed a sharp tightening of the race after Monday’s debate, where Gingrich drew the audience to its feet with a sharp answer about the value of a work ethic.

Romney led by 15 points, 37 percent to 22 percent, among likely voters who were interviewed the day of the debate but by just five percentage points the day after. Other polls have showed Romney’s lead evaporating in South Carolina, as well. In a new Gallup poll, Romney’s lead nationally has settled from a high of 23 percentage points back to 10, still significant but slimmer that it was a week ago.

A Gingrich victory would break a long-standing pattern in Republican races: It would mean the first three contests had been won by three different people, a further sign of how unsettled the contest has been.

Politicians here sense the shift in sentiment. “The wind’s at Newt’s back,” said Sen. Lindsey O. Graham (R), who has not endorsed any of the candidates. Sen. Jim DeMint (R), who four years ago was with Romney but has remained neutral this year, said South Carolina is “clearly a two-man race.”

Adding to Gingrich’s momentum was the decision Thursday by Texas Gov. Rick Perry to quit the race and endorse the former speaker. But still standing in Gingrich’s way as he tries to coalesce conservatives is former Pennsylvania senator Rick Santorum, who in Thursday’s debate questioned Gingrich’s temperament and fitness to lead.

Romney’s campaign has long prepared for a lengthy nomination battle and has superior resources if that becomes necessary. Only a week ago, he was looking to win all three of the initial contests, an outcome that would have put him on a glide path to the nomination. But with Thursday’s announcement that Santorum and not Romney had narrowly won the caucuses in Iowa, Romney is looking at the possibility of being a front-runner who has lost two of the first three contests.

“We have a long process ahead of us — 1,150 delegates to get,” Romney told reporters. “I sure would like to win South Carolina, but I know that if those polls were right, regardless of who gets the final number, we’re both going to get a lot of delegates.”

Romney’s campaign has hedged on whether he will continue to appear on debate stages with his three current rivals, Gingrich, Santorum and Rep. Ron Paul of Texas. Two debates are scheduled next week in Florida — Monday in Tampa and Thursday in Jacksonville.

Gingrich told reporters Friday that he will be in Tampa for the debate and predicted that Romney will, too. “Romney can’t claim that he’s prepared to debate [President] Obama if he’s not prepared to debate Newt Gingrich,” he said.

Romney said during a visit to campaign headquarters in Greenville on Saturday morning that he will participate in the Tampa debate Monday. His chief strategist, Stuart Stevens, told reporters Romney would also participate in the debate in Jacksonville.

“Yeah, yeah, I’ll be doing the debate on Monday,” Romney said in response to a reporter’s question as he worked the phones calling voters at his Greenville headquarters. “Yeah, I’m in.”

In the face of Gingrich’s challenge, Romney brought in re­inforcements, the most significant being Virginia Gov. Robert F. McDonnell (R).

In Baltimore, where House Republicans were gathered for their annual retreat, some influential lawmakers voiced concerns about a prolonged primary fight. House Majority Leader Eric Cantor (Va.) said in an interview that it was important for the party to “coalesce around a single vision with the nominee.”

Cantor, who is unaligned, suggested for the first time that he might endorse. He did not say whom, but he is a close ally of McDonnell.

Romney has been hurt by a halting response about whether he will release his tax returns.

Initially, Romney said he would not release the documents, but by Thursday, he said he would. Asked Friday whether he had erred in his wavering responses, feeding a narrative of him as a “waffler,” Romney said: “I can’t possibly tell you that everything I do in the campaign is perfect.”

Romney tried to turn the tables on Gingrich, calling for the former speaker to release House Ethics Committee documents involving a 1997 reprimand. Asked whether Gingrich should release the documents, Romney said: “Of course he should.”

R.C. Hammond, Gingrich’s spokesman, said the ethics report has long been available and suggested Romney is trying to shift the focus away from the tax issue. “The Ethics Committee spent over two years reviewing all of the stuff that was turned in,” Hammond said.

As he left headquarters and headed to a campaign event at Tommy’s Country Ham House, Romney called on Gingrich to release documents pertaining to his work with Freddie Mac.

“I’d like to see what the report was that he provided to Freddie Mac,” Romney said. “I’d like to see what he advised. He said he was an historian and just provided historical information, then he said he told them what they were doing was somehow not going to work. I’d like to see the report.”

“He also said that he was one of the authors of the Reagan revolution economically and created these jobs,” Romney added. “Now that we’ve looked at the Reagan diaries and seen he’s mentioned only once and in a way where Reagan said he was wrong, I’d like to see what he actually told Freddie Mac. Don’t ya think we ought to see it? This is a big issue. We’ve got Washington insider talking about Freddie Mac, let’s see what his report was to Freddie Mac, what he said to them, what advise he gave them.”

Gingrich was hit Thursday by a potential problem when his former wife Marianne gave interviews in which she said he had asked for an open marriage during a time when he was having an affair with his current wife. Gingrich continued to rebuke the media Friday for its attention to the story. Whether he would be hurt by the latest allegations or helped by a voter backlash against the media wasn’t clear. But Romney aides hoped the issue might rally female voters, who in recent polls have preferred Romney over Gingrich.

Staff writers Philip Rucker and Nia-Malika Henderson and polling director Jon Cohen contributed to this report.