Republican presidential frontrunner Mitt Romney on Monday focused his campaign's firepower on Rick Santorum to head off his surge in two of three states with nominating contests this week.

Coming off a runaway victory in Nevada on Saturday, Romney hopes to cement his front-runner status on Tuesday in caucuses and primaries in Colorado, Minnesota and Missouri - the next battlegrounds in the state-by-state fight to pick a challenger to face Democratic President Barack Obama in November.

Romney's attention has been focused heavily on Newt Gingrich but the former U.S. Speaker of the House's campaign has stumbled and he is no longer a big threat in this week's contests.

Instead, the focus of Romney's well-organized and well-funded attack machine turned to Santorum, a former U.S. senator from Pennsylvania who pipped Romney in the first contest of the 2012 Republican campaign in Iowa but has since faded from view.

Pollsters said Santorum, who has been vying with Gingrich to be the conservative alternative to the more moderate Romney, was leading in Minnesota and was second to Romney in Colorado.

Santorum also has a good chance in Missouri, where Gingrich is not even on the ballot, although the vote there is just symbolic because there are no delegates up for grabs in a non-binding vote.

Wins for Santorum in Minnesota and Missouri would make it clear he's a more viable alternative to Romney than Gingrich and give him a lot of momentum for the road ahead, said Dean Debnam, president of Public Policy Polling.

Underscoring his role as the current threat to Romney, Santorum was second favorite on the Intrade prediction market to win the Republican nomination, just ahead of Gingrich. But Romney is by far the favorite, with an 86.7 percent chance, according to Intrade.

Former Minnesota Governor Tim Pawlenty, a former presidential candidate and now the co-chair of Romney's campaign, criticized Santorum's record in Congress and questioned his conservatism.

He has held himself out by saying he's the 'true conservative.' Pawlenty told a conference call with reporters. He's held himself out as the perfect or near perfect conservative when in fact, that's not his record.

As a U.S. senator, he was a leading earmarker and pork-barrel spender, Pawlenty said, referring to spending projects that lawmakers use to steer federal dollars towards their home districts. He also said when Santorum was in Congress he had voted several times to raise the debt ceiling.

ATTACK MACHINE

Santorum's campaign responded to Pawlenty's statement to reporters and other recent attacks by Romney saying the former Massachusetts governor should run on his record and vision for the future instead of just criticizing his opponents.

But Governor Romney does what he always does and directs his well-funded attack machine to destroy the opponent, said Santorum spokesman Hogan Gidley. Mitt Romney's act is tired, old and wearing thin with voters ....Romney never touts his own record because it's abysmal.

Gidley said Romney's attack and smear campaign is just a way of avoiding his liberal record.

Asked by reporters if the new attacks on Santorum meant that the Romney campaign now saw him as the biggest threat to the nomination, Pawlenty noted that Santorum was poised to do well in the upcoming states.

He's a credible candidate. He deserves to be right in the middle of the back and forth between the campaigns and I think that's what you see happening, Pawlenty said.

But in what seemed to be an effort to already lower expectations for Tuesday, Pawlenty said the Minnesota caucus was generally sparsely attended and those who did show up tended to support the most conservative candidate.

A poll on Monday showed that Obama would win in a match-up against either Romney or Gingrich. The Washington Post-ABC news survey of 1,000 adults found that Obama's prospects have brightened amid a stronger economic outlook and recent job growth.

It said if the election were held now Obama would win 51 percent of the vote compared to 45 percent for Romney. He would win with 54 percent compared to 43 percent for Gingrich.

(Writing by Deborah Charles; Editing by Cynthia Osterman)