Christian conservative Rick Santorum's unexpected trio of state wins has reignited his White House bid and raised new question marks over Republican frontrunner 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 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 frontrunner, with a larger nationwide organization and a heavier war chest than any of his rivals.

But Santorum's triumph 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 Minnesota, Santorum led with 45 percent and Romney was a distant third at 17 percent.

The biggest shock of the night was in Colorado, where Santorum scooped 40 percent of the vote, edging out Romney's 35 percent, according to official party results.

"Wow, what a night for Santorum and a disaster for Mitt," Charles Franklin, cofounder of pollster.com and a professor at Marquette University Law School, told AFP.

"This certainly raises the stakes for Super Tuesday... It's a great second chance for Santorum to replace Newt as the top alternative."

Romney, speaking to supporters in Denver, congratulated Santorum but insisted: "I expect to become our nominee with your help."

Romney and Gingrich will take solace in the fact that none of Tuesday's contests are binding votes, but for Santorum they could bring momentum, media attention and maybe some more cash to fill up his depleted campaign coffers.

"Wow! Conservatism is alive and well in Missouri and Minnesota," Santorum told cheering supporters after victories in those two states.

"Ladies and gentlemen, I don't stand here to claim to be the conservative alternative to Mitt Romney. I stand here to be the conservative alternative to Barack Obama," he said in a victory speech in Saint Charles, Missouri.

Santorum surprised many when he won the Iowa caucuses on January 3, the first stop in the protracted state-by-state primary process that will decide who takes on Obama, a Democrat, in the November 6 general election.

But that was more than a month ago and he badly needed to reset a campaign that flagged through New Hampshire, South Carolina, Florida and last weekend in Nevada, where he finished fourth and last.

With three big victories already under his belt, Romney is still the man to beat, but the former governor of liberal Massachusetts continues to face withering accusations of "flip-flopping" on key conservative issues.

In theory, Tuesday's three states held rich pickings as the four remaining candidates try to secure enough delegates to pocket the total 1,144 needed to secure the Republican nomination.

But the results in Colorado and Minnesota were non-binding, only acting as a guide for later state conventions.

Missouri's vote, meanwhile, was dubbed a "beauty contest," as the state will only award its delegates after a March 17 caucus.

"Since they were non-binding, that blunts it a bit, but it does provide fodder for the narrative that Romney cannot close the deal with the right, particularly with social conservatives," David Damore of the University of Nevada, Las Vegas told AFP.

According to CNN, Romney currently has 106 pledged delegates, Gingrich 38, Santorum 22, and Paul 20.

The Republican establishment hopes it will all be over well before the August 27-31 convention in Tampa, Florida, averting a drawn-out battle that could hurt the eventual nominee's chances against Obama.