COLUMBIA, S.C. - Former House Speaker Newt Gingrich stormed to an upset win in the South Carolina primary Saturday night, dealing a sharp setback to former front-runner Mitt Romney and suddenly scrambling the race for the Republican presidential nomination.

"Thank you, South Carolina!" Gingrich swiftly tweeted to his supporters. He appealed for a flood of donations for the next-up Jan. 31 primary. "Help me deliver the knockout punch in Florida. Join our Moneybomb and donate now," said his tweet.

Romney was unbowed. He vowed to contest for every vote in every state and unleashed a two-pronged attack on President Barack Obama and Gingrich simultaneously.

Referring to criticism of his business experience, Romney said, "When my opponents attack success and free enterprise, they're not only attacking me, they're attacking every person who dreams of a better future. He's attacking you," he told supporters, the closest he came to mentioning the night's primary winner's name.

Already, Romney and a group that supports him were on the air in Florida with a significant television ad campaign, more than $7 million combined to date. Aides to the former Massachusetts governor had once dared hope that Florida would seal his nomination - if South Carolina didn't first - but that strategy appeared to vanish along with the once-formidable lead he held in pre-primary polls.

Former Pennsylvania Sen. Rick Santorum and Texas Rep. Ron Paul trailed badly in the South Carolina voting.

Exit polling showed Gingrich, the former House speaker, leading by a wide margin among the state's heavy population of conservatives, tea party supporters and born-again Christians.

For the first time all year, Romney trailed among voters who said they cared most about picking a candidate who could defeat President Barack Obama this fall. Gingrich was ahead of the field for those voters' support.

As the first Southern primary, South Carolina has been a proving ground for Republican presidential hopefuls in recent years.

Since Ronald Reagan in 1980, every Republican contender who won the primary has gone on to capture the party's nomination.

Romney swept into South Carolina 11 days ago as the favorite after being pronounced the winner of the lead-off Iowa caucuses, then cruising to victory in New Hampshire's first-in-the-nation primary.

But in the sometimes-surreal week that followed, he was stripped of his Iowa triumph - GOP officials there now say Santorum narrowly won - while former Utah Gov. Jon Huntsman dropped out and endorsed Romney and Texas Gov. Rick Perry quit and backed Gingrich.

Romney responded awkwardly to questions about releasing his income tax returns, and about his investments in the Cayman Islands. Gingrich, the former speaker of the House, benefited from two well-received debate performances while grappling with allegations by an ex-wife that he had once asked her for an open marriage so he could keep his mistress.

By primary eve, Romney was speculating openly about a lengthy battle for the nomination rather than the quick knockout that had seemed within his grasp only days earlier.

There were 25 Republican National Convention delegates at stake in South Carolina, but political momentum was the real prize with the race to pick an opponent to Obama still in its early stages.

In all, more than $12 million was spent on television ads by the candidates and their allies in South Carolina, much of it on attacks designed to degrade the support of rivals.

Interviews with voters as they left polling places showed nearly half saying their top priority was finding a candidate who could defeat Obama in the fall, followed by wishes for experience, strong moral character and true conservatism.

In a state with 9.9 percent unemployment, concern about the economy was high, and almost one-third of those voting reported a household member had lost a job in the past three years.

The exit poll was conducted for The Associated Press and the television networks by Edison Research as voters left polls at 35 randomly selected sites. The survey involved interviews with 1,577 voters and had a margin of sampling error of plus or minus 4 percentage points.

Santorum announced shortly after the polls closed that he would open his campaign in Florida today.

Paul has said he intends to skip the state and focus his efforts on caucus contests in Nevada on Feb. 4 and Missouri several days later.

Santorum, a former Pennsylvania senator, pinned his South Carolina hopes on a heavy turnout in parts of the state with large concentrations of social conservatives, the voters who carried him to his surprisingly strong showing in Iowa.

Paul had a modest campaign presence here after finishing third in Iowa and second in New Hampshire. His call to withdraw U.S. troops from around the world was a tough sell in a state dotted with military installations and home to many veterans.

Romney's stumbles began even before his New Hampshire primary victory, when he told one audience that he had worried earlier in his career about the possibility of being laid off.

He gave a somewhat rambling, noncommittal response in a debate in Myrtle Beach last Monday when asked if he would release his tax returns before the primary. The following day, he told reporters that because most of his earnings come from investments, he paid about 15 percent of his income in taxes, roughly half the rate paid by millions of middle-class wage-earners.