The UP isn't as democrat as it used to be. It voted for Dukakis, and later for Bush twice, but still has a lot of its democrat heritage on the state level. Much of that has to do with Dominic Jacobetti and Joe Mack who represented the UP in Lansing. They were the Senator Byrds on the state level.
As for Stupak, he's pro-life, and recently kissed and made up with the NRA so he's popular with them again. On economics, the UP leans center-left. Stupak fits the profile.
Thanks for the information - it all makes more sense now.