Santorum hat-trick in Minnesota, Colorado and Missouri
US presidential hopeful Rick Santorum has swept the contests for the Republican Party nomination in Minnesota, Missouri, and Colorado.
Mr Santorum outperformed longtime front-runner Mitt Romney, who has struggled to connect with the party's conservative base.
Supporters in Missouri heard Mr Santorum declare victory for all those "building the conservative movement".
The eventual nominee will face Barack Obama in November's election.
Former House of Representatives Speaker Newt Gingrich hardly campaigned in the three states that voted on Tuesday, and did not even appear on Missouri's ballot.
Distrust of Romney
In Minnesota's caucuses, with 95% of the vote counted, Mr Santorum was on 45%, while Texas Congressman Ron Paul was on 27% and former Massachusetts Governor Romney had 17%.
In Missouri's primary, with all votes counted, Mr Santorum won with 55%, well ahead of Mr Romney at 25% and Mr Paul on 12%.
After an anxious wait, the Republican Party chairman in Colorado eventually declared Mr Santorum the winner in that state's caucuses, too.
Final results showed Mr Santorum won the state with 40% of votes, with Mr Romney on nearly 35%.
Pitching himself as the only true conservative in the race, Mr Santorum had campaigned hard in Minnesota and Missouri - states with significant blocs of Tea Party and evangelical Christian voters respectively.
Polls had showed him performing well, and predicted the possibility he would win in either or both states. But while Mr Romney's team had sought to manage expectations, they still retained hopes of a Colorado victory.
"Conservatism is alive and well in Missouri and Minnesota," he said before the Colorado results were known. "I don't stand here to be the conservative alternative to Mitt Romney. I stand here to be the conservative alternative to Barack Obama."
The former Pennsylvania senator, who had not won a contest since his narrow win in Iowa's caucuses in January, had been viewed as a long-shot candidate.
Tuesday's victories will inject new momentum into his campaign, as he hopes to displace Mr Gingrich as the Mr Romney's main challenger.
Mr Gingrich, who was campaigning in Ohio, told CNN: "I think the big story coming out tonight is going to be that it's very hard for the elite media to portray Governor Romney as the inevitable nominee after tonight's over."
Correspondents say Mr Gingrich's game plan is to ride out February and hang on until March when Southern states come into play. As a former Georgia representative with a long history in the south, his campaign feels he stands a better chance of success in those states.
In a last-ditch effort to win over social conservatives ahead of Minnesota and Colorado's caucuses, Mr Romney tried to boost his credentials on being anti-abortion, pro-religious freedom and opposed to gay marriage.
During his first run for the Republican presidential nomination back in 2008, when he challenged John McCain, Mr Romney won in both Colorado and Minnesota.
But both states are perceived to have moved to the right since then, so doubts over his Mormon faith and political record as governor of a liberal state could have cost him votes.
Playing down the significance of Tuesday's contests, Mr Romney told supporters in Denver: "This was a good night for Rick Santorum. We'll keep on campaigning down the road, but I expect to become our nominee with your help.
"When this primary season is over, we're going to stand united as a party behind our nominee to defeat Barack Obama."
Thirty-seven delegates were at stake in Minnesota and 33 Colorado, although they are not officially awarded until later this year. The primary in Missouri is being dubbed a "beauty contest" since it will actually allocate its delegates via a caucus held next month.
Before Tuesday's votes, Mr Romney had 101 of the 1,144 delegates needed to clinch the nomination at the Republican Party convention in August, according to an Associated Press news agency tally.
In second place, Mr Gingrich was on 32 delegates, Mr Santorum 17 and Mr Paul nine.