Michigan has an open primary. While you can’t vote in both parties’ primaries, there is nothing to stop you from voting in a primary you care about rather than one where the results are foreordained. For example, in 2008 I cast a Republican ballot for John McCain because I thought he was a generally decent guy and I really don’t like Mitt Romney. (As you may recall, Obama wasn’t on the ballot in Michigan’s sham of a 2008 primary).
There is some talk that Democrats may vote en masse for Santorum in order to keep an internecine Republican primary going. A Michigan loss would look horrible for Romney. On the other hand, if Romney wins in Michigan and Arizona, he’ll start looking inevitable again. But maybe rogue Democrats can tip Santorum over the edge in Michigan?
I’m skeptical. Because could you really bring yourself to vote for Santorum? Even if it’s for a good cause? McCain was grumpy and backwards. I could vote for grumpy and backwards over Romney. But Santorum? What will you tell the grandkids?