US Republican presidential candidate Rick Santorum trained his sights on rival Mitt Romney after his shock three-state vote sweep, pressing his case that he can take his underdog campaign all the way to the White House.
Written off only a few weeks ago, Santorum won contests in Minnesota, Colorado and Missouri - the latest turnaround in the volatile race to pick a candidate to challenge President Barack Obama in the November elections.
While Tuesday's startling defeats did not derail Romney's campaign, they put the Republican frontrunner on notice that he is not invincible and raised questions about his conservative appeal.
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"You know, I believe conservatives are beginning to get it, that we provide the best opportunity to beat President Obama," Santorum told CNN Wednesday.
Santorum had been seen surging in the Midwestern states of Minnesota and Missouri thanks to support from evangelical Christians, but few expected him to win in the Rocky Mountain west.
It was a bitter blow for Romney, who had romped home in Colorado and Minnesota during his 2008 bid with large leads in the final counts.
While the triple win catapulted Santorum at least for the moment past former House speaker Newt Gingrich into the role of Romney's main rival, the ex-senator from Pennsylvania insisted his wins were not just symbolic.
"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," he said in his victory speech.
He also dismissed a statement from the Romney campaign saying he has neither the funds nor the political machinery needed to mount a national campaign.
"We're doing very, very well raising money. I think last night we raised a quarter of a million dollars online," Santorum said.
The latest contests could now reposition the Republican battle ahead of "Super Tuesday" on March 6, when 10 states vote at once in the state-by-state nominating process.
Romney goes into that day as the man to beat, but Santorum's triumphs put fresh pressure on the former governor of liberal Massachusetts still seeking to convince the party base of his conservative bona fides.
"I don't know if Santorum's wins ... constitute a major upset, but they are surprising and do show just how soft Romney's support is," political science professor John Brehm of the University of Chicago told AFP.
Obama's Democratic allies also gleefully seized on the disarray in the Republican camp as it fails to coalesce around one candidate.
Pointing to a low turnout in Tuesday's votes as reinforcing "the growing narrative of an enthusiasm gap on the Republican side," the Democratic National Committee said in a statement that "primary voters have been confronted with a set of deeply-flawed candidates."
Santorum was the big winner in Missouri's primary, with 55 per cent of the vote, more than double Romney's take. In the Minnesota caucus, Santorum led with 45 per cent and Romney was third, behind small-government crusader congressman Ron Paul.
The shock of the night was Colorado, where Santorum beat Romney by five percentage points in the caucus.
