1 of 4
The Denver Post, Aaron Ontiveroz) MANDATORY CREDIT; MAGS OUT; TV OUT, Associated Press
Mitt Romney speaks during a rally party at Arapahoe High School in Centennial, Colo., on Monday, Feb. 6, 2012.

GOLDEN, Colo. — Republican presidential challenger Rick Santorum is hoping that his weeks of criticism of GOP front-runner Mitt Romney will pay off Tuesday when Colorado and Minnesota hold nominating caucuses. Romney, in turn, is looking to continue his winning streak after back-to-back victories in Florida and Nevada last week.

The outcome of Colorado and Minnesota isn't likely to dramatically change the dynamics of the GOP presidential race. But a Santorum victory in either of them could give him a boost — for a day at least — while shining a light on Romney's troubles with conservative voters who long have been skeptical of his candidacy.

"This is a big day for us to see whether all the work that we put in in the past few weeks, when not so much attention was paid here as to Florida and Nevada, pays off," Santorum said at a Denver rally a night before the contests got under way.

Santorum, a former Pennsylvania senator and fierce anti-abortion rights opponent, bypassed Nevada and Florida to essentially camp out in Colorado and Minnesota, spending the past week assailing Romney to lay the groundwork among conservatives who dominate. Santorum has portrayed himself as the only conservative choice in those caucus states.

In recent days, Romney has sensed a Santorum threat and has sought to prove that he, too, has strong conservative stances on social issues, despite a history of reversing himself on abortion and gay rights.

Newt Gingrich and Ron Paul also are on ballots in Colorado and Minnesota, but neither has aggressively competed in the states.

The latest step in the month-old nomination fight comes as President Barack Obama's campaign is asking top fundraisers to support a Democratic-leaning outside group that is backing the president's re-election bid as it works to compete with the tens of millions of dollars collected by Republican-backed outside groups in the presidential race.

It's a reversal for Obama, who has long been opposed to "super" political action committees, which can spend unlimited amounts of cash to influence elections. And it comes at a time when Romney is in a position of strength, after his allied groups successfully beat back challenges by opponents early in the primary season.

Heading into Tuesday, Romney leads in the hunt for delegates to the Republican nominating convention this summer, with 101, while Gingrich has 32. Santorum and Paul trail with 17 and 9 respectively, according to The Associated Press count. Colorado has a total of 36 delegates, including 33 up for grabs while Minnesota has a total of 40 delegates, with 37 at stake. Missouri has no delegates at stake as it holds what amounts to a beauty contest. Instead, Missouri will award its 52 delegates through a system of caucuses and conventions, starting March 17.

In Minnesota, Eric Radtke, 32, was looking for the party's best hope to defeat incumbent Democratic President Barack Obama in the fall and said he planned to vote for Romney because of that.

"Every time I hear him he seems to exude the level of respect for this country that I think it deserves," said Radtke, a telecommunications salesman from Shakopee, Minn.

Terry Groetken couldn't disagree more. The 71-year-old retired optical salesman from Plymouth, Minn., said he would have to "hold my nose" to vote for Romney in November if he wins the nomination because he doesn't trust him to stay consistent on core conservative principles. Groetken planned to cast his vote for Gingrich, saying "He's a bulldog."

On Monday night, Santorum continued his criticism of Romney, calling him an unfit candidate and warning that Republicans were heading toward defeat if Romney were the nominee.

"A Romney nomination would not be in the best interest of us wining the general election and we need to have a conservative alternative. And my feeling is that Speaker Gingrich sort of had his chance in the arena and came up short in Florida and Nevada," Santorum told reporters in Golden on Monday.

Earlier, Romney went after Santorum's time in the Senate as "not effective" because of his past support for spending on pork-barrel projects. Santorum flatly countered that Romney was "uniquely unqualified."

Hunt reported from Grand Junction, Colo. Shannon McCaffrey in Colorado and Brian Bakst in St. Paul, Minn., contributed to this report.