Today is caucus day for Colorado and Minnesota, and there's a non-binding primary in Missouri.
Mitt Romney is hoping to extend his delegate lead and prove he's the most electable.
Still, while Romney is focusing his attacks on the president, he's taking fire from his fellow Republicans.
Even though no delegates are on the line in Missouri, Rick Santorum is going head to head with Romney there, and some polls show promise for the former Pennsylvania Senator.
Santorum is hoping to emerge as the conservative alternative to Romney and change the dynamics of this race in a single day.
Ron Paul and Newt Gingrich, meanwhile, are in a battle for delegates, hoping to stretch out the nomination fight as long as they can.