AP

Newt Gingrich has stormed to an upset victory over Mitt Romney in the South Carolina GOP primary with a pair of strong debate performances that most voters said was important to their choice.

Within minutes, the former speaker of the House was looking ahead to the next contest Jan. 31.

"Thank you South Carolina!," he tweeted. "Help me deliver the knockout punch in Florida."

Even as he congratulates Newt Gingrich on his victory in the South Carolina primary, Mitt Romney is telling supporters that Gingrich isn't the one to lead the Republicans to victory in November.

He says Republicans can't win with a candidate who has never run a business and has never run a state. Without singling out Gingrich by name, Romney again criticized Republican opponents who he said had joined Democrats in attacking the free enterprise system. Gingrich had been among those raising questions about Romney's tenure as head of a venture capital firm.

In exit polls, most voters said they made up their minds in the last few days of the campaign. Two-thirds said the debates were an important factor in their vote.

Gingrich led among the state's conservatives, tea party supporters and born-again Christians. Romney held a small advantage among moderate and liberal voters, and those who are not born-again Christians.

Romney vowed to "keep fighting for every vote" in the weeks and months ahead. He's already looking ahead to the next primary in Florida, a state that he says has "suffered terribly" under what he calls the "failed policies" of Obama.

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