Newt Gingrich wins the South Carolina Republican primary with a convincing victory over Mitt Romney. The result blows the Republican race for the presidential nomination wide open.

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Gingrich's come-from-behind win in conservative South Carolina injects unexpected volatility into a Republican nominating race that until this week appeared to be a coronation for Mitt Romney.

Gingrich's triumph may lead to a protracted battle for the Republican nomination in which the candidates spend money and energy to beat each other instead of having the party unite behind one standardbearer focusing on Obama.

With nearly all the votes counted, Gingrich had pulled in 40 per cent of the vote, followed by Romney with 28 per cent. Former senator Rick Santorum was in third with 17 per cent and US congressman Ron Paul in fourth with 13 per cent.

Riding a series of feisty debate performances, the former speaker of the House of Representatives captured the lingering unease of conservative voters in South Carolina who view Romney's moderate past and shifting policy stances with suspicion. Gingrich argued that he would be able to better articulate the party's conservative ideals.

The next contest is the Florida primary on 31 January.

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Newt Gingrich (Reuters)

'I don't shrink from competition'

South Carolina was a stunning turnaround for Gingrich, whose campaign barely survived after top staff quit last June and stumbled to a disappointing finish just three weeks ago in Iowa, the first Republican nominating contest.

He finished fourth in both Iowa and New Hampshire, the next contest, as conservatives split their votes among several candidates.

Gingrich contrasted his sometimes-chaotic management style with Romney's buttoned-down approach, arguing that his campaign was powered by ideas rather than logistics. Romney is one of the wealthiest candidates ever to run for president and his campaign is well financed.

"We don't have the kind of money that at least one of the candidates have. But we do have ideas and we do have people," Gingrich told supporters in a 22-minute tirade against Obama, the news media, judges and other "elites."

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Romney, acknowledging that there will be a "long primary season," said he would continue to run on his business record and paint Gingrich as a creature of Washington in the weeks ahead.

"I don't shrink from competition, I embrace it," Romney told supporters. "I believe competition makes us all better. I know it's making our campaign stronger."

Obama, who does not face a primary challenger, will have his turn in the spotlight on Tuesday with his State of the Union address. In a message to supporters on Saturday, he said the speech would focus on "building an economy that works for everybody, not just a wealthy few."