At National Review, Byron York tries to get to the bottom of why Romney won Michigan Republican primary. I think it has less to do with Romney's promises regarding the auto industry which I didn't hear about (or maybe didn't remember) until this morning and more about the number of and frequency of his TV commercials.
I think I've seen about 5 different Romney spots in the past month. The first commercial attacked Hillary Clinton as not having the experience to run government and noted Romney's experience as businessman, at the Olympics and as governor of Massachusetts. Another commercials touted his prolife/pro-family positions, another notes his endorsements from some Michigan newspapers, one featured a man sharing how Romney helped find his daughter in New York and another hit on Romney's Michigan roots and talked about our one-state recession.
I saw only two McCain commercials - one discussing his status as a troublemaker for Washington (noting campaign finance reform, pork-barrel spending, and his disagreement with Rumsfield's strategy) and another which shared quotes from various Michigan newspaper endorsements. McCain's commercials aired fairly frequently but not as often as Romney's.
Huckabee had a single commercial which I saw around 5 times which discussed his prolife/pro-family beliefs. I probably saw a commercial from the Club for Growth which attacked Huckabee's record on taxes as many times (if not more) as I saw Huckabee's ad.
Romney appealed to differing segments of Michigan's population, McCain appealed to individuals with independent streaks and Huckabee appealed to socially conservatives voters.
With those kind of appeals, these kinds of results aren't that surprising. Romney's support was broad-based, McCain won among individuals who are independents and Democrats, don't attend church, have no religion, are dissatisfied with Bush and Huckabee did well with individuals who attend church often and think the religious beliefs of a candidate matter a great deal.