Rick Santorum has won Minnesota’s Republican caucus, giving him a second big win on Tuesday night and adding to the headache for Mitt Romney and his hopes of quickly wrapping up the Republican presidential nomination.

Mr. Santorum’s victory in Minnesota — a state that Mr. Romney won easily in 2008 — came shortly after he was declared the easy victor in Missouri, where he trounced his rivals in the Republican primary.

He is also leading in early returns in Colorado’s Republican caucus.

Mr. Santorum’s surge came just in the last several days after Mr. Romney celebrated a hard-fought victory in Nevada and sought to turn his attention away from his Republican rivals and instead toward President Obama, as the party’s likely nominee.

But that approach changed dramatically in the last 24 hours as his campaign started attacking Mr. Santorum in e-mails to reporters. Tim Pawlenty, the former governor of Minnesota and a supporter of Mr. Romney’s, on Monday accused Mr. Santorum of not being “ready” to be president.