Chief correspondent

Just a week ago, what was most striking about the Republican presidential race was the possibility that the party’s least-dominant front-runner in many years — Mitt Romney — could effectively wrap up the GOP nomination faster than anyone in his party ever had. That came crashing down Saturday night.

Newt Gingrich’s stunning victory in South Carolina, which came after he finished fourth in Iowa and fifth in New Hampshire, respectively, rewrites in dramatic fashion the latest story line of the Republican campaign. Now a competition that, for all practical purposes, might have ended here moves on to Florida for another major showdown on Jan. 31. In all probability, the fight will continue well beyond that.

Florida presents a major challenge, given its size and complexity. The stakes there will be sizable. Romney cannot afford another defeat there, given that he has more resources to wage a campaign in a state where campaigning is costly. Gingrich, however, risks losing his momentum if he is not able to capitalize on this success in Florida.

Though Romney is clearly hurt, many Republicans still see him as the favorite to win the nomination. The overriding question is whether Saturday’s loss is merely a small setback of the kind experienced by many past presidential nominees. Or does South Carolina mark the beginning of real erosion in Romney’s standing that could lead to the former House speaker winning the nomination, something unthinkable only a month ago?

That’s what Florida — and then Nevada, Michigan, Arizona and perhaps other states on the calendar — will tell us. But there is no doubt that the defeat here on Saturday represents a setback to Romney, who now has won just one of three opening contests in the GOP race after it looked like he was positioned to start the year 3-0 (though he still has two second-place finishes and thus the best overall record of the field). Strong debate performances by the former House speaker and a week of missteps and stumbles by the former Massachusetts governor brought the race to this moment.

A week ago there were six candidates still standing in the GOP race. Now, though technically there are four — the others being former Pennsylvania senator Rick Santorum and Rep. Ron Paul (Tex.) — the Republican race is now the two-person contest that many once anticipated. That it is between Romney and Gingrich is the latest evidence of conventional wisdom being thrown into the wastebasket.

In a head-to-head race, Romney enjoys superior resources and a superior campaign operation. His campaign long has prepared for a protracted contest. Gingrich is riding the momentum of someone who twice went through near-death experiences, overcame the odds and emerged ready to fight on.

Romney has the support of elected officials and what passes for a Republican establishment, many of whom see Gingrich as a risky nominee. Gingrich is trying to tap the energy of the conservative grassroots of a party whose base stands to the right of Romney and has never been comfortable with him.

South Carolina underscored what has been known about Romney for a year, which is that he has not won the hearts of Republican voters. His appeal was that many nonetheless saw him as the party’s best hope to defeat President Obama in the general election, and it is still the key to his hopes of winning the nomination.

In South Carolina, however, Gingrich won a slim majority among the 45 percent of the electorate here that cited electability as the most important candidate quality in their vote. No doubt the debates played a critical role in those assessments. If that carries over to other states, the dynamic of the race may have changed fundamentally.

A Gingrich adviser, who spoke only on the condition of not being named in order to assess the state of the campaign, said that if the former House speaker has crossed the threshold of acceptability on the question of electability, he could defeat Romney on issues and ideology.

South Carolina also exposed other problems for Romney, which is that he has not been an effective candidate in the closing days of a race. Though he won New Hampshire handsomely, he had two bad days there just before the primary. Here in South Carolina, there were more unforced errors, particularly the stumbling way he dealt with calls for him to release his tax returns.

He will need a quick pivot to address that weakness. But Romney’s advisers said Saturday that they welcome a two-person race with Gingrich and that the former governor is both ready and eager for the next phase of the contest. They said the contrast between a businessman and former governor versus a two-decade member of Congress who has been a Washington fixture for decades would work to Romney’s benefit.

“Once you’re down to a one-on-one race, then it’s a binary choice and we like that choice,” said Russ Schriefer, a top Romney adviser.

Drawing those contrasts began with Romney’s concession speech on Saturday night. He cast Gingrich, though not directly by name, as a rival who has never run a business or a state and who has attacked free enterprise. He suggested that Gingrich has picked up “the weapons of the left” to try to bring Romney down and said that someone “who demonizes success is not fit to be our nominee.” That will be part of the core message he takes into Florida beginning Sunday.

For Gingrich, the question is whether South Carolina will be remembered as his high water mark. His confidence grew by the day over the last week, beginning with his debate performance Monday and aided by a second strong debate Thursday. By the day before the primary, it was evident that he believed he would win here and was now in a position to challenge Romney seriously for the nomination.

But it has often been the case that Gingrich makes mistakes when he is riding high and that is the danger he faces now. Call it overconfidence, hubris or whatever, Gingrich has been his own worst enemy in the past — and he will come under an even louder barrage from the GOP establishment who see Romney as a more reliable candidate to lead the party in the fall.

Romney has the money and organization that give him a head start in Florida. Nearly 200,000 Floridians have already cast ballots in the primary, and several GOP strategists said it is likely that Romney leads among that group.

But superior campaign money and organization count for less in this campaign than in some past ones. Debates have done much to supplant TV ads and direct mail as the most effective medium for delivering messages and for voters to assess the candidates. In South Carolina, nearly two-thirds said debates were either the most important or one of several important factors in their choice. Among that group, Gingrich won by better than 2-1.

All that makes Florida as unpredictable as everything else in this race has been. After Florida comes Nevada and Michigan — both states that Romney carried in his unsuccessful 2008 bid for the nomination — and Arizona. That gives the Romney camp some hope that they can absorb the lessons of Saturday’s defeat, rebound and eventually prosper.

But thanks to the voters in South Carolina, who delivered another memorable primary campaign, Romney now faces a real test of his capacity to rally his party and fend off a competitor who long ago was written off.