Rick Santorum had a breakthrough night Tuesday, winning GOP presidential contests in Missouri, Minnesota and Colorado, all of which is expected to breathe life into his struggling campaign and slow Mitt Romney’s march to the Republican presidential nomination.

The Santorum triumphs promise to, at least temporarily, alter the face of the campaign going into the crucial “Super Tuesday” contests, as the caustic tone of the primaries is expected to continue and intensify. Romney and his allies have signaled that they will use their financial advantage to launch stepped-up attacks on Santorum and on former House speaker Newt Gingrich, the other main challenger.

Santorum solidly defeated Romney in Minnesota and Missouri, and he narrowly edged the former Massachusetts governor in Colorado, according to state GOP officials.

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The victories mark a sharp turnaround for the former Pennsylvania senator, whose candidacy had been sputtering after he failed to capi­tal­ize on his narrow win in Iowa last month. Santorum’s wins across the Midwest Tuesday could bestow new legitimacy on his insurgent efforts and boost his fundraising in the critical period before next month’s major contests.

Santorum now appears to pose a more serious threat not only to Romney, but also to Gingrich, who had been positioning himself as the logical alternative to Romney.

Santorum staked his own claim on Tuesday.

“Conservatism is alive and well,” he told supporters at his election night party in Missouri. “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.”

For Romney, his poor showing Tuesday raised anew the question that has dogged his candidacy all along: Can the relatively moderate, former Massachusetts governor become an acceptable standard-bearer of a party that is increasingly dominated by evangelical conservatives and tea party activists who have long been skeptical of Romney?

After big wins in Florida and Nevada, Romney had hoped to extend his winning streak as he moved to strengthen his claim to the mantle of presumptive nominee. But in recent days, he was clearly bracing for losses on Tuesday.

Romney enjoyed strong establishment backing in Minnesota, with the vocal support of former governor Tim Pawlenty, yet he trailed not just Santorum but also Rep. Ron Paul (Tex.), finishing a distant third.

Addressing supporters in Denver, Romney congratulated Santorum and insisted that he still expects to eventually become the nominee.

“We’re going to take our message of liberty and prosperity to every corner of the country, and when this primary season is over, we’re going to stand united as a party behind our nominee to defeat Barack Obama,” Romney said.

Paul, who has yet to secure a win in the 2012 sweepstakes, mobilized his loyal support networks in the three states and was banking on low turnout in the nonbinding contests to give him a win. But he finished third in Missouri and was trailing in Colorado.

Gingrich did not compete in Missouri and spent limited time in Colorado and Minnesota. He looked past Tuesday’s contests and instead campaigned in Ohio, one of several delegate-rich states voting on Super Tuesday.

Together, the three states voting Tuesday will eventually award 128 delegates. But Missouri was a “beauty contest” with no delegates at stake, while Minnesota and Colorado were nonbinding events with delegates to be chosen this spring.

At stake Tuesday night was the prestige of winning. And Santorum nailed down three upsets to restore an air of viability to his candidacy.

By defeating Romney, Santorum believes, he can reset the race and help create the perception that he, and not Gingrich, is the conservative alternative to the establishment front-runner.

“I feel great that Minnesota is going to change the direction of this race tonight,” Santorum told supporters at a Tuesday afternoon rally in Blaine, Minn. “Governor Romney’s been able to bully his way through this primary . . . but he’s not inspiring. He’s not painting the vision.”

Recognizing the Santorum threat, Romney and his supporters attacked the former senator the past few days over his support for congressional earmarks. Romney shifted his efforts in recent days to Colorado, where he has a deeper ground organization and won the 2008 caucuses with 60 percent of the vote.

The Minnesota and Missouri contests were early barometers of Romney’s support in some of the key Midwestern states he would need to win in a general election against President Obama. And Colorado is thought of as a critical battleground; Obama’s campaign advisers consider it a must-win.

The candidates spent considerably less money and time in these three states than in the contests that preceded them in Iowa, New Hampshire, South Carolina, Florida and Nevada; now the race enters a February lull. After the Maine caucuses, which will end on Saturday, there are no primaries or caucuses until primaries on Feb. 28 in Arizona and Michigan, two states that will generate intense engagement from all the candidates. Romney, in particular, is planning to compete aggressively in both.

Romney officials labored to preserve an aura of inevitability around his campaign, releasing a strategy memorandum Tuesday promoting the former Massachusetts governor’s financial and organizational advantage over the long haul.

“Mitt Romney is the only candidate with the organizational strength and broad-based appeal to secure delegates in all remaining primaries and caucuses,” national political director Rich Beeson wrote. “Of course, there is no way for any nominee to win first place in every single contest — John McCain lost 19 states in 2008, and we expect our opponents to notch a few wins too.”

Henderson reported from Ohio.