Rick Santorum won unexpected victories Tuesday in three U.S. Republican Party nominating contests, reviving his challenge to frontrunner Mitt Romney.

"Wow. Conservatism is alive and well in Missouri and Minnesota," the staunchly conservative Santorum told supporters after he won both those two states. "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."

Santorum was also a projected winner in Colorado early Wednesday. With 89 percent of the vote counted he enjoyed 38.6 per cent support, compared to Romney's 35 percent.

The victories raised new doubts about Romney's ability to attract the conservative base of the Republican party, and recast Santorum as a credible alternative to Newt Gingrich as the Tea Party champion.

Prior to Tuesday's votes former Massachusetts governor Romney had been the clear frontrunner after decisive wins in Florida and Nevada last week.

Gingrich did not compete in Missouri, where the vote had been written off by some as a "beauty contest" with no actual delegates to the Republican convention being awarded and a separate caucus due to be held later for that purpose.

Gingrich came fourth in Minnesota and was third in Colorado, whileCongressman Ron Paul came second in Minnesota and was fourth in Colorado.

The outcome sets up an intriguing three-way contest for "Super Tuesday" on March 6, when voters in 11 states cast their votes.

Santorum, a former Pennsylvania senator, had previously notched a surprise win in the Iowa caucuses, which kicked off the election season last month. His latest victories will help him raise cash to continue fighting Romney and Gingrich for the Republican nomination.

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