The former Pennsylvania senator's wins in the Minnesota and Colorado caucuses and Missouri primary were setbacks for Romney, the national front-runner, who had been expected to cruise easily through a series of relatively minor February voter tests. He must now wait three weeks to regroup, when Arizona and Michigan hold what suddenly are shaping up as unexpectedly important primaries.
In remarks to delirious supporters in St. Charles, Mo., Santorum took a swipe at Romney's big advantage in money and the negative ads he's used to defeat his opponents in previous states. He also lashed out at President Obama, describing him as someone "who thinks he knows better" and doesn't listen to the American people.
"Ladies and gentlemen, 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 said, setting off chants of "We pick Rick!"
Reflecting what he sees as the altered shape of the race, Santorum told CNN that "now we're in a little bit of a no-man's land" as the candidates move into states where they haven't had months, or years, to campaign.
Romney, who won Minnesota four years ago, was running a weak third behind Santorum and Ron Paul.
Speaking to a deflated crowd of backers in Denver, Romney said he was "pretty confident" he would come in either first or second in Colorado's caucuses, his last hope for salvaging a bad night. But hours later, state Republican Chairman Ryan Call announced over CNN that Santorum had won.
He congratulated Santorum and said that he looked forward to coming contests and a united party when the primaries ended. But he also struck a pose as a populist outsider, speaking of his father's humble roots and casting himself as the antidote to the problems in the nation's capital.
"Washington will never be reformed by those who have been compromised by the culture of Washington," he said, noting that he had "never served a day" there. And in another shot at his three remaining rivals, he added that leadership is about starting a business, "not getting a bill out of committee."
But his parting line — "we've got a long way to go" — spoke to the changed circumstances of the GOP contest. Many analysts had said that the race was over, a conclusion not supported by the results late Tuesday.
Santorum's rise poses a serious threat to Newt Gingrich, who had hoped to emerge as the conservative alternative to Romney. The former House speaker fared poorly as Republicans cast ballots in three states, and he is in serious danger of slipping behind Santorum in the GOP race and the fight to be Romney's main foe.
Romney, still the favorite for the GOP nomination, had managed to put together back-to-back successes with wins in Florida and Nevada. Santorum, whose last victory came more than a month ago in Iowa's leadoff caucuses, had his best showing since that day.
Paul, the only candidate without a victory, has been targeting caucus states, including the two voting Tuesday. Delegates in both states will be apportioned later, and Missouri's primary has no connection to delegate selection.
"We're well on our way, and we're going to keep our momentum," the Texas congressman, who is hoping to win his first state when Maine announces the results of its caucuses on Saturday, told supporters late Tuesday in suburban Minneapolis.
In Colorado and Minnesota, voters were gathered at precinct caucuses, where straw polls were conducted to reflect attendees' presidential preferences.
Tuesday's caucuses were conducted under the same process as those in Iowa, which took weeks to sort out. Romney initially was said to be the winner, but after state party officials checked ballots, Santorum was declared the winner, defeating Romney by 34 votes out of more than 121,500 cast.
The Missouri primary provided Santorum with his clearest opportunity yet to go head-to-head with Romney as the conservative alternative, since Gingrich had failed to qualify for the statewide ballot. Gingrich and Santorum have competed to eliminate the other and become the ultimate choice of the party's most ardent conservatives against Romney.
But their cause — stopping the front-runner — is likely to have a better chance of succeeding if both Santorum and Gingrich remain viable candidates, at least for now.
Recent primary polling has indicated that if one of them were to drop out, at least a portion of his support would go to Romney, allowing the former Massachusetts governor to gain delegates more quickly than if the anti-Romney vote continued to be divided among several candidates.
Turnout in Missouri was reported to be low, and the candidates had spent little time campaigning there since the election was a "beauty contest" with no bearing on delegates to the national convention.
State law required the primary to take place, but because of its early timing, Republican National Committee rules would have penalized Missouri for using it to select delegates. As a result, the state party opted to pick them in a separate caucus process that starts next month.
Even before the primary season began, Romney strategists were concerned about maintaining momentum this month because of the dearth of binding contests.
The Romney camp moved early Tuesday to lower expectations and said the candidate was prepared to wage a "methodical, long-haul campaign," with more money and organization than either Santorum or Gingrich had.
A memo, sent out from the Boston headquarters as voting was underway in Missouri and soon to begin in the other states, said there was "no way for any nominee to win first place in every single contest," and "we expect our opponents to notch a few wins, too."
"Romney is the only candidate prepared to compete in simultaneous campaigns across the country," Rich Beeson, the Romney campaign's political director, said in the memo. As for the others, "even 'success' in a few states will not mean collecting enough delegates to win the nomination."
Gingrich, in a tacit acknowledgment that he did not expect to do well Tuesday, spent the day campaigning in Ohio. The Midwest state is one of 11 that will vote on March 6, known as Super Tuesday.
The former House speaker, seeking to turn his candidacy around that day, needs a strong showing in Ohio — the largest Super Tuesday state and one of the biggest general-election battlegrounds — to bolster his argument that he still has a chance to become his party's nominee.