Rick Santorum's taken a large lead in Michigan's upcoming Republican primary. He's at 39% to 24% for Mitt Romney, 12% for Ron Paul, and 11% for Newt Gingrich.
Santorum's rise is attributable to two major factors: his own personal popularity (a stellar 67/23 favorability) and GOP voters increasingly souring on Gingrich. Santorum's becoming something closer and closer to a consensus conservative candidate as Gingrich bleeds support.
An interesting data point -- Democrats and Independents are supporting Santorum as well:
Santorum's benefiting from the open nature of Michigan's primary as well. He's only up by 12 points with actual Republican voters, but he has a 40-21 advantage with the Democrats and independents planning to vote that pushes his overall lead up to 15 points.