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 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."

Steve Peoples covers national politics and the presidential campaign for the Associated Press.