COLUMBIA, S.C. — Newt Gingrich has won the South Carolina GOP primary, CNN projects, with Mitt Romney finishing second.

Gingrich’s projected victory marks a stunning turnaround for a campaign that observers had left for dead — again — just weeks ago.

Less than a week ago, Romney, the New Hampshire winner, was looking at a double-digit lead in most polls of likely voters in South Carolina’s primary, a big lead in Florida and the possibility of a clear path to the GOP nomination. But Gingrich turned in two strong debate performances in the state this week while Romney was put on his heels by his rivals.

In his victory speech to supporters, Gingrich thanked “everyone in South Carolina who decided to be with us in changing Washington.”

“It is very humbling and very sobering to have so many people who so deeply want their country to get back on the right track — so many people who are so concerned about jobs, about medical costs, about the everyday parts of life, and who feel that the elites in Washington and New York have no understanding, no care, no concern, no reliability and in fact do not represent them at all,” Gingrich said.

Gingrich, whose surge has been helped in part by two strong debate performances in South Carolina, said that if he’s resonating with voters, it’s not just because he’s a good debater.

“It’s not that I am a good debater,” he told his supporters Saturday night. “It’s that I articulate the deepest felt values of the American people.”

The winner of the South Carolina primary, which this year is the third contest on the primary and caucus calendar, has gone on to win the Republican nomination in every election since 1980.

With the victory, Gingrich becomes the third GOP candidate to win an early GOP contest this year. It is the first time since 1980 that three different GOP candidates have won nominating contests in Iowa, New Hampshire and South Carolina.

Gingrich’s turnaround is just his latest. He came out of nowhere to top national polls in late fall on the strength of debate performances, but dropped again ahead of the Iowa caucuses as opponents hammered him in Iowa ads.

“Gingrich has been harder to kill than Rasputin,” Republican strategist and CNN contributor Alex Castellanos said Saturday. “He has been dead three times in this campaign, and … the guy keeps coming back.”

Romney congratulated Gingrich in a speech to supporters and said he would continue fighting for the nomination.

“Our campaign has fought very hard here in South Carolina, and in the coming weeks and months, I’ll keep fighting for every single vote. I will compete in every single state,” he said. “We’re going to win this nomination and we’re going to defeat President Obama in November.”

Texas Rep. Ron Paul, who along with former Pennsylvania Sen. Rick Santorum rounds out the surviving GOP primary field, told supporters he expected to get four to five times as many votes in South Carolina as he did four years ago.

“So there’s every reason to be encouraged,” he said.

Santorum congratulated Gingrich on what he called an “amazing victory” and vowed to continue campaigning.

“The great narrative is that three days ago, there was an inevitability in this race,” Santorum said. “Mitt Romney was 2-0 (before Iowa officials certified the narrow Iowa result this week, which showed Santorum actually ahead by 34 votes) and soon to be 3-0, and I took Iowa, Newt took South Carolina and it’s game on again.”

Santorum said he was preparing not only to campaign in Florida, where the next primary awaits on Jan. 31, but in other states, indicating he believed he had what it took to sustain his campaign for a long fight. Romney has held a large lead in polls of likely primary voters but recent polls show the race tightening a bit there, too.

“This is a long race. As a result of what happened here tonight, this race isn’t going to be over next week or the week after,” Santorum said. “This is going to be a long one, and it’s going to be the best thing that could happen for whoever the eventual Republican nominee is, because that nominee is going to be sharpened by steel.”

Two weeks ago, Romney’s campaign was looking at two wins under its belt, a big lead in South Carolina, a bigger lead in Florida and the possibility of a clear path to the Republican presidential nomination.

As late as Tuesday, Romney had a double-digit lead in most polls of likely voters in the state’s primary, but Gingrich turned in the two strong debate performances in the state this week while Romney was put on his heels by his rivals.

Then what had been declared an eight-vote Romney victory in Iowa’s January 3 caucuses was reversed into a 34-vote win for Santorum when the state party certified its results on Thursday.

Later that day, Texas Gov. Rick Perry suspended his campaign and threw his support to Gingrich.

“It has been a hard week,” state treasurer Curtis Loftis, a leading Romney surrogate, said Friday. “Nobody is going to deny that.”

Gingrich had been building up momentum all week. A poll released Saturday morning showed the former House speaker’s surge over the last week carrying him past Romney, who had been the front-runner in the state all month. The American Research Group poll shows Gingrich leading Romney by a 40 percent to 26 percent margin. ARG’s last poll, released Thursday, showed a virtual tie with Gingrich at 33 percent and Romney at 32 percent.

Gingrich and Romney both campaigned in the conservative Upstate on Saturday with Gingrich presenting himself as the conservative alternative to the “Massachusetts moderate” Romney while Romney continued to attack Gingrich as he has over the past week as polls tightened.

At his Greenville campaign headquarters, Romney launched a new line of attack, calling for Gingrich to release details on his work for government-backed mortgage giant Freddie Mac, an institution unpopular with conservatives.

“Didn’t he say he was going to release information about his relationship there?” Romney asked. “Let’s see what report he wrote for Fannie Mae and Freddie Mac, what his conclusions were and what the contract looked like. I thought he said he was going to do that.”

Romney’s campaign also has been pressing the former House speaker to release the full report from a 1990s ethics investigation that led to his downfall in Congress. Gingrich was reprimanded by the House and ordered to pay a $300,000 penalty in 1997 for violating an ethics rule.

This week, Romney’s campaign sent Gingrich a cake marking the 15th anniversary of that reprimand, according to a Romney campaign source. A picture provided by that source shows the wording “Happy 15th anniversary, Mr. Speaker! … Now release the records,” written in icing on the cake.

An anticipated run-in between the two front-runners didn’t materialize at a Greenville restaurant where both had booked events at the same time. Romney showed up about 45 minutes early and had left before Gingrich arrived.

Upon arriving, Gingrich asked, “Where’s Mitt? I thought he was going to stay and maybe we’d have a little debate this morning.”

— CNN’s Jim Acosta, Adam Aigner-Treworgy, Dana Bash, Peter Hamby, John Helton, Jason Hanna, Shawna Shepherd and Paul Steinhauser contributed to this report.