According to Sunday evening's PPP tracking poll, 16% of Michigan's likely GOP primary voters had already early voted by mail. Of that 16%, 62% had cast their votes for Mitt Romney, and 29% had cast their votes for Rick Santorum. If PPP's numbers were right, then before the polls opened on Tuesday morning, Romney had roughly a built-in 5-point lead on Santorum (10 points to 5 points).
Romney ended up besting Santorum in Michigan by a 3-point margin: 41% to 38%. So if PPP's likely voter model for Michigan was correct, then Santorum bested Romney among those votes that were cast on Tuesday, because Santorum narrowed Romney's 5-point lead to a 3-point lead. Romney still won, though, because the campaign had banked enough absentee votes to serve as a cushion against the Santorum insurgency.
I can't say it for sure, but it looks to me like the Romney campaign's early voter aspect of its campaign in Michigan was the difference-maker. If Romney hadn't had such an early voting advantage when the polls opened on Tuesday morning, then it would certainly have been very close on Tuesday night.
Absentee vote campaigns are the kind of thing where organization and state official endorsements really make it happen. Romney had both. Santorum had neither.