Rick Santorum jolted the Republican presidential race Tuesday with a three-state sweep of nominating contests in Colorado, Missouri and Minnesota, puncturing Mitt Romney's claim to be the unstoppable front-runner.

Mr. Santorum's victories—one in the Mountain West and two in the Midwest—gave his campaign a much-needed burst of momentum while stirring doubt about Mr. Romney's abilities to woo conservatives to his side in important electoral swing states such as Missouri and Colorado. So far, Mr. Romney has won in states where he has devoted significant campaign time and advertising dollars.

In two of the states, Colorado and Minnesota, Mr. Romney had won by large margins in 2008. He was hugely favored in Colorado in the days leading up to the Tuesday caucus. Two of his top aides have deep experience in the state, and his campaign has been busy organizing there for months.