About 1 in 10 Michigan voters were Democrats and they backed Santorum, a former Pennsylvania senator, by about a 3-1 margin. During the Michigan campaign, Romney accused Santorum of using automated telephone calls to attract Democrats to his candidacy, which Santorum answered by saying Romney had sought independent support in earlier state contests.
No indication here how many were strategic voters—voters supporting Obama who wanted to help weaken the GOP field/eventual nominee. I’d guess very few.
UPDATE: Aaron Blake writes “Operation Hilarity Fizzles.”