WASHINGTON (AP) -- A resurgent Rick Santorum won Republican presidential caucuses in Minnesota and Colorado on Tuesday night, a stunning sweep that raised fresh questions about front-runner Mitt Romney‘s ability to appeal to the ardent conservatives at the core of the party’s political base.

Santorum, a former Pennsylvania senator, triumphed, as well, in a nonbinding Missouri primary that was worth bragging rights but no delegates to the party‘s national convention.

Romney, the former Massachusetts governor, prevailed in both Minnesota and Colorado in 2008, the first time he ran for the nomination, but the Republican Party has become more conservative in both states since then.

The victories by Santorum, a fierce and vocal opponent of abortion and gay rights, exposed Romney’s longtime struggles to convince cultural conservatives that he‘s now in line with their beliefs despite his previous support of gay and abortion rights.

Sensing likely defeat, the former Massachusetts governor’s team had worked to lower expectations in the run up to voting in the three states and the candidate himself had started emphasizing his positions on social issues in the days since he won Saturday‘s Nevada caucuses. It was a clear sign that he sensed a threat from Santorum.

The victories were the first for Santorum since he eked out a 34-vote win over Romney in the leadoff Iowa caucuses a month ago.

Santorum had faded far from the lead in the primaries and caucuses since Iowa, and Newt Gingrich, the former speaker of the House of Representatives, seemed to eclipse him as the leading conservative rival to Romney when he won the South Carolina primary late last month.

The results were grim for Gingrich, who made scant effort in any of the states that voted during the day. He ran far off the pace in the two caucus states, forced to watch from the sidelines while Santorum boasted of being the candidate with conservative appeal.

Before the Colorado results were announced, a jubilant Santorum declared to cheering supporters in St. Charles, Missouri: "Conservatism is alive and well in Missouri and Minnesota!’‘

Challenging his rival, he declared that on issues ranging from health care to "Wall Street bailouts, Mitt Romney has the same positions as Barack Obama.’‘

Returns from 91 percent of Minnesota’s precincts showed Santorum with 45 percent support, Texas Rep. Ron Paul with 27 percent and Romney -- who won the state in his first try for the nomination four years ago -- with 17 percent Gingrich trailed with 11 percent.

It was closer in Colorado, where returns from all the precincts showed Santorum with 40 percent of the vote to 35 percent for Romney. Gingrich had 13 percent, and Paul claimed 12 percent.

With all the precincts counted in Missouri, Santorum had 55 percent of the vote, followed by Romney with 25 percent. Paul had 12 percent, while Gingrich wasn‘t on the ballot.

There were 37 Republican National Convention delegates at stake in Minnesota and 33 more in Colorado, and together, they accounted for the largest one-day combined total so far in the race for the Republican nomination. Missouri’s delegates to the party‘s national nominating convention will be chosen in caucuses beginning next month.

Santorum campaigned aggressively in all three states holding contests Tuesday, seeking a breakthrough to revitalize a longshot campaign that had struggled since Iowa. He won Minnesota largely the way he did neighboring Iowa, dispatching his organizers from the first state to the second and courting evangelical pastors and leaders of the tea party movement which advocates limited government and low taxes.

But Santorum remains a longshot for the nomination because Romney has an overwhelming advantage in campaign funds, organizational strength and support from the party establishment needed to sustain what is now even more likely to be a drawn out state-by-state battle for the nomination.

It was not clear where Santorum could exploit his victory. Aides have already said he has little hope in Maine caucuses that end this weekend, the next event on the calendar.

Romney showed no sign of disappointment with Tuesday’s results in remarks to supporters.

"This was a good night for Rick Santorum. I want to congratulate Sen. Santorum, but I expect to become the nominee with your help," he told supporters in Denver.

Republican Sen. Roy Blunt of Missouri, a Romney supporter, congratulated Santorum, but added "Mitt Romney has the organization and the resources to go the distance in this election."

Romney, despite victories in New Hampshire, Florida and Nevada, has struggled since 2008 to convince some cultural conservatives that he‘s conservative enough. The former Massachusetts governor, who runs as the Republican most likely to defeat Obama, is still trying to establish his credentials among social conservatives suspicious of a one-time moderate who backed abortion rights.

To counter Santorum, Romney shifted his focus from the economy to social issues, raising such issues as abortion, religious freedom and gay marriage in recent days, part of an intensified effort to win over social conservatives in states voting Tuesday.

Romney attacked the Obama administration’s recent decision to require Catholic organizations to provide contraceptive aids in some circumstances and later pounced after a federal appeals court ruled that a voter-approved ban on gay marriage in California violated the U.S. Constitution.

"Remarkably under this president‘s administration there is an assault on religion -- an assault on the conviction and religious beliefs on members of our society,’‘ Romney, a Mormon, told supporters in Loveland, Colorado.

As Romney stepped up his emphasis on social issues, Santorum and rival Gingrich intensified their criticism of Romney on those same issues.

In an op-ed published Tuesday in Politico, Santorum seized on Romney’s 2005 decision to require all Massachusetts hospitals, including Catholic ones, to provide emergency contraception to rape victims. Some Catholics say the so-called morning-after pill is a form of abortion.

"He said then that he believed `in his heart of hearts‘ that receiving these contraceptives -- free of charge -- trumped employees’ religious consciences," said Santorum, a Catholic. "Now, a few years later and running for president, his heart is strategically aligned with religious voters opposing this federal mandate."

Santorum‘s victories in Minnesota and Colorado gave him at least 28 delegates, pushing him past Gingrich into second place in the delegate count. Romney got at least six delegates.

Overall, Romney has 107 delegates, including endorsements from members of the Republican National Committee who automatically attend the party’s national convention and can support any candidate they choose. Santorum has 45 delegates, Gingrich has 32 and Paul has nine.

The next major contests in Michigan and Arizona are scheduled for Feb. 28. Then comes March 6, or Super Tuesday, when 10 states hold nominating contests with 416 convention delegates at stake. Georgia, where Gingrich launched his career in Congress, is the biggest prize that night with 76 delegates. Next is Ohio, which has 63 delegates at stake and where early voting has already begun.

Gingrich spent the day campaigning in Ohio, where early voting has already begun.

His campaign went into a downward spiral after he won the South Carolina primary in an upset. The former speaker was routed in the Florida primary to Romney, then finished a distant second in Nevada over the weekend.