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WASHINGTON — A resurgent Rick Santorum won Minnesota's Republican presidential caucuses with ease Tuesday night and was declared the victor in Colorado as well, a stunning sweep that raised fresh questions about front-runner Mitt Romney's appeal among the ardent conservatives at the core of the party's political base.

Santorum triumphed, as well, in a nonbinding Missouri primary that was worth bragging rights but no delegates.

"Conservatism is alive and well in Missouri and Minnesota," the jubilant former Pennsylvania senator told cheering supporters in St. Charles, Mo. Challenging both his GOP rival and the Democratic president, he declared that on issues ranging from health care to "Wall Street bailouts, Mitt Romney has the same positions as Barack Obama."

Romney, a former Massachusetts governor, was expected to remain the front-runner for the GOP nomination nevertheless, thanks to his huge advantages in campaign cash and organization going forward, and his impressive earlier wins in New Hampshire, Florida and Nevada.

Still, the solid Santorum vote provided fresh evidence that "Romney's is a troubled candidacy," said Lawrence Jacobs, a professor of political science at the University of Minnesota. "The outcome of the race is far from certain."

The strong showing by Santorum made it clear that Romney isn't yet his party's consensus nominee. It signaled that the GOP nomination campaign may remain a bitter struggle for months, possibly leading to a divided August convention and a weakened candidate against President Barack Obama in the fall.

In Minnesota's caucuses, with 83 percent of precincts reporting, Santorum, a former U.S. senator from Pennsylvania, had 45 percent. Texas Rep. Ron Paul had 27 percent, and Romney trailed with 17 percent. Newt Gingrich, former Speaker of the House of Representatives, had 11 percent. Romney won the Minnesota GOP caucuses in 2008.

In Missouri, a crucial swing state in the November elections, Santorum was headed for a landslide. With 99 percent of precincts reporting, he had 55 percent to Romney's 25 percent. Paul had 12 percent. Gingrich was not on the ballot; 4 percent were uncommitted.

The Missouri vote was a non-binding "beauty contest," since no delegates were at stake and candidates made little effort to campaign there. But Santorum's victory gave his campaign renewed momentum.

Santorum, speaking to supporters in St. Charles, Mo., contrasted himself to Obama.

"I don't stand here to claim to be the conservative alternative to Mitt Romney," he said. "I stand here to be the conservative alternative to Barack Obama."

Paul, whose strategy centers on caucus states, said he was pleased with his second-place showing in Minnesota. "Our views are not only being accepted, they're being sought after," he told backers in Golden Valley, Minn.

Romney addressed supporters in Denver as Colorado results trickled in. "This was a good night for Rick Santorum," he said, but added, "I expect to become the nominee with your help.'

No delegates were chosen Tuesday. Caucuses in Colorado and Minnesota began a process that will lead to delegate selection there in April and May. Missouri's primary is a "beauty contest" only; its 52 delegates will be chosen in state and local conventions later this spring. Tuesday's contests were about influencing public opinion and building momentum.

Turnout in the three states appeared to be low; only about 60,000 voters turned out for Minnesota's caucuses in a state that now has about 3 million registered voters. About half the voters in Missouri and Minnesota were expected to be Republicans who consider themselves conservative Christians.

Romney hoped to sustain the momentum he'd gained with big wins in Florida and Nevada over the past week. But many conservatives remain uncomfortable with him. A Pew Research Center survey in late January found that 52 percent of Republicans rated the GOP presidential candidate field fair or poor.

Should conservatives rally around Santorum _ or take fresh looks at Gingrich or Paul _ no candidate is likely to amass the 1,144 delegates needed for the GOP nomination quickly.

Romney had pinned his hopes Tuesday on Colorado's caucuses. He canceled a planned stop Monday in Minnesota so he could campaign in Colorado, and he was to host a post-caucus rally Tuesday night in Denver.

He attempted some damage control earlier Tuesday, releasing a memo from political director Rich Beeson.

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"As our campaign has said from the outset, Mitt Romney is not going to win every contest," Beeson said. He pointed out that 2008 GOP nominee John McCain lost 19 states in the nominating season that year, "and we expect our opponents will notch a few wins, too."

Santorum campaigned as the faith and family conservative, arguing that Romney is too moderate. Santorum has appealed to voters at churches, touted his long-standing opposition to abortion and blasted Romney for requiring Massachusetts residents to obtain health care insurance coverage.

Gingrich has tried to woo this crowd, but his personal past _ notably two divorces and extramarital affairs _ is "a problem with some activists," said Charles Slocum, a former Minnesota Republican Party chairman. Gingrich campaigned Tuesday in Ohio, which votes March 6.

The next contests are in Maine, which concludes its caucuses Saturday, and Arizona and Michigan, which hold primaries Feb. 28. Romney remains favored to win all three, and he appears well-positioned to string together more wins on Super Tuesday, March 6, when 10 states vote and more than 437 delegates are at stake.