WASHINGTON • Rick Santorum took advantage of his opportunity Tuesday to become the chief conservative alternative to Mitt Romney.

Romney, a former Massachusetts governor, was expected to remain the front-runner for the GOP nomination whatever happened Tuesday, thanks to his huge advantages in campaign cash and organization, and his impressive earlier wins in New Hampshire, Florida and Nevada.

A solid Santorum vote in Missouri, Minnesota and Colorado on Tuesday provided fresh evidence that "Romney's is a troubled candidacy," said Lawrence Jacobs, a professor of political science at the University of Minnesota. "The outcome of the race is far from certain."

The strong showing by Santorum, a former U.S. senator from Pennsylvania, made it clear that Romney isn't yet his party's consensus nominee. It signaled that the GOP campaign may remain a bitter struggle for months, possibly leading to a divided August convention and a weakened candidate against President Barack Obama in the fall.

Turnout in the three states was low; only about 60,000 voters turned out for Minnesota's 2008 GOP caucuses, in a state that now has about 3 million registered voters. About half the voters in Missouri and Minnesota were expected to be Republicans who consider themselves conservative Christians.

Romney had hoped to sustain the momentum he'd gained with big wins in Florida and Nevada over the past week. But many conservatives remain uncomfortable with him. A Pew Research Center survey in late January found that 52 percent of Republicans rated the GOP presidential field fair or poor.

Should conservatives rally around Santorum — or take fresh looks at Gingrich or U.S. Rep. Ron Paul of Texas — no candidate is likely to quickly amass the 1,144 delegates needed for the GOP nomination.

Romney pinned his hopes Tuesday on Colorado's caucuses. He canceled a planned stop Monday in Minnesota so he could campaign in Colorado, and he was to host a postcaucus rally Tuesday night in Denver.

He attempted some damage control earlier Tuesday, releasing a memo from political director Rich Beeson. "As our campaign has said from the outset, Mitt Romney is not going to win every contest," Beeson said. He pointed out that 2008 GOP nominee John McCain lost 19 states in the nominating season that year, "and we expect our opponents will notch a few wins, too."

Santorum campaigned as the faith and family conservative, arguing that Romney is too moderate. Santorum has appealed to voters at churches, touted his long-standing opposition to abortion and blasted Romney for requiring Massachusetts residents to obtain health insurance coverage.

Gingrich has tried to woo this crowd, but his personal past — notably two divorces and extramarital affairs — is "a problem with some activists," said Charles Slocum, a former Minnesota Republican Party chairman. Gingrich campaigned Tuesday in Ohio, which votes March 6.

The next contests are in Maine, which concludes its caucuses Saturday, and Arizona and Michigan, which hold primaries Feb. 28. Romney remains favored to win all three.