Republican White House hopeful Newt Gingrich's shock victory in South Carolina's primary on Saturday makes him the sharpest conservative threat to long-time favourite Mitt Romney.

The pugnacious former House Speaker ignited conservative passions fuelled by feisty debate performances to torch the former Massachusetts Governor and multi-millionaire investor's vaunted campaign riches and well-oiled machine.

Mr. Gingrich, repeatedly declared politically dead over the past year, must now harness his political momentum heading into the vital battleground of Florida, a more diverse state where Mr. Romney's weapons were expected to give him the edge.

Mr. Gingrich cast his insurgent victory here as a coup for Americans “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.”

The result here was a heavy symbolic blow to Mr. Romney: No Republican since 1980 has won the nomination without carrying this conservative bastion, now home to prominent leaders of the “Tea Party” movement.

And it was the first time that different candidates have carried the first three nominating contests — Iowa, New Hampshire, and South Carolina — highlighting the volatile nature of the race.

But “beyond the media impact, and some momentum, the fundamentals — money, organisation, and endorsements signifying establishment support — still favour Romney,” said Matt Dickinson, a political scientist at elite Middlebury College.

Still, after benefiting from seeing conservative voters fractured among several candidates, Mr. Romney could face a real threat if Mr. Gingrich manages to “establish himself as the anti-Mitt,” Mr. Dickinson told AFP.

Over the past year, conservatives have rallied at various points behind candidates sometimes dubbed “not-Romney,” only to see them flame out, while Mr. Romney has struggled to push his support above 25 per cent of Republicans.

“This race is getting to be even more interesting,” Mr. Romney told cheering supporters packed into a room dressed up as though for a victory rally before congratulating Mr. Gingrich for “a hard-fought campaign.”

“I don't shrink from competition. I embrace it. I believe competition makes us all better. I know it's making our campaign stronger,” he added, as his campaign readied to travel to Florida on Sunday.

There were signs Mr. Romney's once-substantial advantage at the national level may also be shrinking, with pollsters Gallup saying on Friday that his U.S.-wide lead over Mr. Gingrich plummeted to 30-20 per cent.

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