Republican presidential hopeful Rick Santorum scored a major victory on Tuesday by beating Mitt Romney in three Midwest nominating contests, signalling trouble ahead not just for the front-runner but also for Newt Gingrich.

The stunning hat trick for Santorum, a staunch social conservative with a penchant for sweater vests, saw him more than double Romney in the Missouri primary, with unofficial results suggesting he took 55 per cent of the vote compared to Romney's 25 per cent.

He also handily won the Minnesota caucuses, with unofficial results suggesting he took 46 per cent of the vote. Libertarian congressman Ron Paul placed second at 26 per cent, Romney took 16 per cent and Gingrich brought up the rear with just 11 per cent of the vote.

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Republican presidential hopeful Rick Santorum listens to a questioner during the Colorado Election Energy Summit on Monday. Santorum campaigned aggressively in all three states holding votes Tuesday. (Stan Honda/AFP/Getty Images)

And he managed to eke out the narrowest of victories over Romney in the Colorado caucuses, a contest the former Massachusetts governor was favoured to win.

Santorum took about 38 per cent of the vote compared to Romney's 37 per cent. Gingrich and Paul essentially tied for third at 13 and 12 per cent respectively.

Santorum was gleeful during his Missouri victory speech in St. Charles, marvelling at his Romney shellacking.

"Conservatism is alive and well in Missouri and Minnesota. We doubled him up here and in Minnesota!" he said to his cheering supporters.

He took sharpest aim at U.S. President Barack Obama, however, pushing red-hot conservative buttons by telling the crowd: "He thinks he knows better. He thinks he's smarter than you. He thinks he should be able to rule over all of you."

Santorum added he wasn't satisfied "to stand here to be the conservative alternative to Mitt Romney. I stand here to be the conservative alternative to Barack Obama."

Romney gracious in defeat

Romney was gracious in his defeat to Santorum, congratulating his rival before turning his attention to Obama in a concession speech in Colorado.

"Under his own definition, Obama has failed. We will succeed," he said.

The Missouri contest has been dubbed a "beauty contest" primary, one that nonetheless turned ugly for Romney. It won't send any delegates to the Republican convention until the state's March 17th caucuses are held.

Gingrich didn't qualify to get on the ballot in Missouri, and Romney didn't spend much money there.

But Missouri was considered a barometer of the sentiments of primary voters, unaffected by a barrage of negative advertising in the state that was a hallmark of previous contests.

Santorum's win gives his campaign a much-needed shot of adrenalin and will undoubtedly lead to an increase in fundraising for the candidate, who has lagged well behind his rivals in terms of cash.

Indeed, while all three contests held Tuesday are technically meaningless, they are nonetheless viewed as a serious momentum-booster heading into the crucial next few weeks of the campaign. In this year's topsy turvy Republican race, however, momentum has often evaporated like ether.

The results also show Romney still has a major problem on his hands connecting with the party's base, who clearly harbour serious misgivings about the front-runner. But he wasn't the only one rebuked by Santorum's triumphs in the socially conservative Midwest — the results suggest the one-time senator may be nudging Gingrich out of the way as the favoured candidate of far-right conservatives.

Next primaries on Feb. 28

The next primaries are not until Feb. 28, and Santorum argued Tuesday night that his strong showing in the Midwest proves he's the best conservative alternative to Romney.

Public Policy Polling, the pollster that put Santorum in front in both Missouri and Minnesota, says he does well with the Midwest's Tea Party activists, evangelicals and voters who describe themselves as "very conservative."

"While Romney and Gingrich have hammered each other in recent weeks, Santorum's been largely left alone and he's benefiting from that now," the polling company's Tom Jensen wrote on the PPP website.

"It appears right-leaning Republican voters are shifting toward Santorum as their primary alternative to Romney."

Santorum narrowly beat Romney in last month's Iowa caucuses, the official launch of the Republican race.

But following his Iowa triumph, he had disappointing finishes in New Hampshire, South Carolina, Florida and Nevada, unable to make good on his attempts to convince social conservatives that he's a better alternative to Romney than Gingrich.

Gingrich, in fact, has repeatedly suggested that Santorum should drop out of the race to allow him to consolidate the social conservatives, Tea Party activists and evangelicals among the party's primary voters.

The likeability factor

Santorum has refused, boldly predicting over the weekend that he would ultimately prevail to win the party's nomination as his likeability factor began to outpace his rivals' among primary voters.

Behind the scenes, some Republican establishment social conservatives — dead set against Gingrich — have apparently been encouraging Santorum to stay in the race.

Santorum and his backers have been encouraged by a recent Rasmussen poll that suggests he'd have the best chance of beating Obama in November's presidential election.

Conservative pundits, commentators and news outlets are also now publicly rallying behind him.

A story on the National Review's website on Tuesday asked: "Again, Why Not Santorum?" Bill Kristol of the Weekly Standard also suggested voters in Colorado, Minnesota and Missouri give Santorum the opportunity to "replace Gingrich down the road as the alternative to Romney."

Santorum's big night came less than a month before so-called Super Tuesday on March 6, when 10 states hold voting contests with more than 200 delegates up for grabs.