U.S. Republican presidential candidate Mitt Romney is hoping to solidify his front-runner status in the race for his party's nomination with strong showings in Tuesday's caucuses in Colorado and Minnesota.

Romney is holding a rally Tuesday in Colorado, where he appears poised to win. But polls in Minnesota have suggested former Pennsylvania senator Rick Santorum could take first place there. Santorum has campaign events Tuesday in Colorado, Minnesota, and Missouri.

Former House of Representatives speaker Newt Gingrich, once considered Romney's top rival for the nomination, held a large lead in the Colorado polls late last year. But he has since lost ground in that state and was focusing his attention Tuesday on Ohio, as the state begins early voting ahead of its primary on March 6.

Romney won Colorado and Minnesota when he unsuccessfully ran for the Republican presidential nomination in 2008.

Texas Representative Ron Paul holds an election event in Minnesota Tuesday night.

The caucuses are part of the state-by-state process to pick a Republican challenger to take on U.S. President Barack Obama, a Democrat, in the November election. There is also a primary Tuesday in Missouri, but it is non-binding and no delegates are at stake.

On Monday, the four main candidates campaigned in Minnesota and Colorado. Romney focused his message on Santorum to try to head off any momentum produced by the former senator.

Romney is coming off huge victories last week in the Florida primary and the Nevada caucuses. On Saturday in Nevada, he tallied 50 percent of the vote compared with 21 percent for his closest challenger, Gingrich. Santorum and Paul finished far behind.

Gingrich has vowed to fight all the way to the Republican nominating convention in Florida in August. It is early in the selection phase of the Republican fight to win back the presidency. To be selected to face Mr. Obama, a Republican needs to have the support of 1,144 delegates at the August nominating convention. To date, Romney has an estimated 101 committed delegates, Gingrich 32, Santorum 17 and Paul 9.

In a hypothetical election match-up, a new poll indicates that President Obama would beat Romney. The ABC News-Washington Post survey shows Mr. Obama ahead 51 percent to 45 percent among registered voters .

The poll found that half of those surveyed approve of President Obama's job performance and believe he deserves a second term.