MINNEAPOLIS, Minnesota - Christian conservative Rick Santorum's unexpected trio of state wins has reignited his White House bid and raised new question marks over Republican front-runner Mitt Romney.

Santorum, written off only a few weeks ago, won caucuses in Minnesota and Colorado and a primary in Missouri on Tuesday - a clean sweep that represented another stunning turnaround in this topsy-turvy Republican presidential race.

The established wisdom was that Santorum was surging in the Midwest and could take Minnesota and Missouri thanks to support from evangelical Christians, but no one expected him to win out west in the Rocky Mountains.

It was a bitter blow for Romney, who romped home in Colorado during his 2008 bid, scooping more than 60 percent of the vote.

"The Romney bandwagon just went in the ditch," CNN analyst David Gergen said, as pundits scratched their heads and struggled to explain the loss.

The latest contests could reposition the Republican battle to be the nominee to take on US President Barack Obama ahead of "Super Tuesday" on March 6, when 10 states vote at once and almost a fifth of all delegates are decided.

A clutch of seven February contests, including the three held on Tuesday, will not alter the fact that Romney goes into that day the front-runner, with a larger nationwide organization and a heavier war chest than any of his rivals.

But Santorum's sweep puts added pressure on the favorite, threatening to unite a party base that still doubts Romney's conservative bona fides.

The surge by Santorum, a former US senator, arguably places him back out in front of former House speaker Newt Gingrich, whose campaign has slumped in recent weeks, making a mockery of his claim to be the obvious "anti-Mitt".

Santorum was the big winner in Missouri's primary, with 55 percent of the vote, more than double Romney's 25 percent.

In the northern state of Minnesota, Santorum won 45 percent of the vote, easily defeating Texas congressman Ron Paul at 27 percent. Romney was a distant third at 17 percent, with 95 percent of precincts reporting.

Romney just can't shake his difficulty attracting conservatives. And that reality is undercutting the former Massachusetts governor's effort to cast himself as the inevitable Republican presidential nominee and prolonging a race that each day exposes deep divisions within the party.

Newt Gingrich also now faces a fresh challenge to his claim that he's the chief conservative alternative to Romney, the Republican front-runner.

AFP-AP