Well it's NOT his home state. Massachusettes is. But it was the Romney campaign that labelled Michigan as his home state, so he really screwed himself there.
He was born and grew up there -that by definition suggests Michigan is in fact his home state.
I thought he was born in Philadelphia...
I don't consider where you "grow up" to be your political home state. I consider it to be where you spend the last 10-or-so years of your life. Maybe where you were last elected, but if you've been out of office for a while, it's where you've been living.