CNN and NBC called victories for Santorum in Minnesota and Missouri -- sucking some of the wind out of the campaign sails of Mitt Romney, Newt Gingrich and Ron Paul -- and he was leading in the early going in Colorado where the outcome was still up in the air.
With 73.85 percent of the caucuses reporting in Minnesota, results posted by the secretary of state's office put Santorum ahead with 44.65 percent to 27.4 percent for Paul, 17.02 percent for Romney and 10.63 percent for Gingrich.
With 3,121 of 3,134 precincts reporting in Missouri's primary, the secretary of state posted results showing Santorum with 55.2 percent of the vote, followed by Romney at 25.3 percent and Paul at 12.2 percent. Gingrich was not on the ballot in the Show-Me State. Uncommitted votes totaled 4 percent and several other one-time GOP hopefuls received less than 2 percent each.
In Colorado, CNN reported that with 30 percent of the caucuses counted, Santorum led with 42 percent to 30 percent for Romney, followed by Gingrich with 15 percent and Paul at 12 percent.
"Conservatism is alive and well in Missouri and Minnesota," Santorum told cheering supporters outside St. Louis.
"I don't stand here and claim to be the conservative alternative to Mitt Romney. I stand here to be the conservative alternative to Barack Obama."
Heading into the day's electoral events, Romney had charged that Santorum's time as a U.S. senator from Pennsylvania from 1995 through 2006 was "not effective" because he aggressively spent taxpayer dollars on pork-barrel projects. But polls had shown Santorum surged in Colorado and Minnesota, threatening Romney's winning streak.
Romney, the front-runner in this primary campaign season, won both Minnesota and Colorado four years ago.
Santorum, meantime, questioned Romney's conservative credentials and said the former Massachusetts governor "should not be our nominee" because he was "dead wrong" in signing Massachusetts' healthcare reform in 2006.
The reform mandates nearly every Massachusetts resident obtain a minimum level of healthcare coverage. The law served as a model for the national healthcare overhaul President Barack Obama signed into law two years ago.
The Romney camp aggressively confronted Santorum on his healthcare accusations Monday, issuing a "research briefing" alleging Santorum made "false attacks on Massachusetts healthcare," which the Romney campaign said was different from the national overhaul.
Gingrich, a former House speaker from Georgia who was trailing Santorum in Colorado and Minnesota opinion polls, said he was the only Republican candidate who could defeat Obama in November.
"We have to set the stage to have a campaign this fall in which we have a team running so we can win," Gingrich said at a rally in Golden, Colo., citing his work in the House during the Reagan and Clinton administrations as credentials.
Paul, the libertarian-leaning Texas congressman, has said he would do better in caucus states and he also campaigned in Missouri, where state delegates to the Republican National Convention in August will be determined in state caucuses March 17.
Caucuses are also going on all week in Maine.