DENVER (AP) -- Mitt Romney just can't shake his difficulty attracting conservatives. And that reality is undercutting his effort to cast himself as the inevitable Republican presidential nominee and prolonging a race that each day exposes deep divisions within the party.

Newt Gingrich also now faces a fresh challenge to his claim that he's the chief conservative alternative to Romney, the GOP front-runner.

Those were the big takeaways from Rick Santorum's surprise victories Tuesday night in Colorado, Minnesota and Missouri - which, for now at least, keep his struggling candidacy alive.

"Conservatism is alive and well in Missouri and Minnesota," the former Pennsylvania senator told cheering supporters 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."

Santorum broke a four-state losing streak by successfully pitching himself as the only true conservative in Tuesday's races.

The results focus attention on Romney's and Gingrich's weaknesses, while underscoring the degree to which the GOP primary battle is likely to stretch well into the spring and perhaps even the summer. The outcomes also are likely to detract from Republicans' efforts to lambaste President Barack Obama.

While Santorum may get a short-term boost of momentum, it's unclear whether the cash-strapped candidate has the resources to capitalize quickly on the wins and compete against Romney's national political machine.

Santorum is a candidate with a post office box as a national headquarters. He's using volunteers to handle his scheduling. And he has virtually no staff to help turn momentum into votes in the critical Super Tuesday contests that are now four weeks away.

His rivals face problems of their own.

Romney has struggled to win over conservatives, who for years have viewed him skeptically for his shifts and reversals on issues they hold dear, like abortion and gay rights.

Romney hadn't lost a nomination fight since his second-place finish in South Carolina 17 days ago. He went on to comfortably win the Florida primary and Nevada's caucuses. And polling in those two states suggested that Republicans of all stripes - social conservatives, tea party activists and those in the mainstream - had finally begun to set aside doubts about his conservative credentials.

Romney used that, and his back-to-back victories, to his advantage. He had started to portray himself as the presumptive nominee as establishment Republicans rallied behind him.

But in recent days, Romney sensed a Santorum surge in Colorado, Minnesota and Missouri, and started to court social conservatives with get-tough positions against abortion and gay rights as he worked to convince them that he was pure on key issues despite his more moderate positions of the past.

It didn't work - and he recognized as much in a brief speech to a partially empty room of supporters gathered in Denver as the results came in.

"This was a good night for Rick Santorum," a more subdued Romney said. But he added, "We'll keep on campaigning down the road, but I expect to become our nominee with your help."

Gingrich, for his part, did his best to ignore one of his worst days in the campaign, trailing far behind Santorum and Romney.

The former House speaker spent the day campaigning in Ohio and staying out of sight when results rolled in from Minnesota, Colorado and Missouri. He had no immediate comment on an outcome that put into question his standing in the race.

The first day of multi-state voting in the GOP race exposed a glaring deficiency for Gingrich: He lacked the resources and organization to compete. He's trying to project strength heading into a series of Super Tuesday elections on March 6, but his decision to barely campaign in the trio of states this week gave Santorum the opportunity to suggest that he's once again become the anti-Romney candidate.

Both Romney and Gingrich clearly knew it would be a bad day.

Romney political director Rich Beeson released a memo early Tuesday that noted that Arizona Sen. John McCain lost 19 states on the way to capturing the GOP presidential nomination in 2008.

And, on Monday night, Gingrich predicted, "I think that Santorum's going to have a pretty good day." Gingrich also started making excuses for a poor showing.

"I stayed in Florida to fight it out," Gingrich said. "He took the same amount of time and energy and he came to Minnesota and Missouri and Colorado. For him, that was the right decision."

The race now moves to Maine, whose low-profile caucuses conclude Saturday, before heading to Arizona and Michigan. Romney is poised to do well in Michigan, given his family ties to the state his father once governed.

Ultimately, history may reveal that Tuesday's results were little more than an embarrassing blip for a Romney campaign that holds massive advantages over underfunded and under-organized rivals. But it may take months to find out.

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EDITOR'S NOTE - Steve Peoples covers national politics and the presidential campaign for The Associated Press.

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