I'm planning on writing a longer-length story about the Motor City and what its decline means for American politics. I've always had sort of a morbid fascination with Detroit, partly because I love cars, partly because it's extraordinary to see a great city fall so far, and partly because I love history and it's interesting to see some aspect of a city frozen in time.
With regards to pure electoral politics, I think that Michigan will just get more Democratic. The people who have left Detroit are socially mobile young people, many of whom are white. Many of them are also Republican. The folks sticking around in the urban core tend to be part of the core Democratic constituency. Detroit is perhaps as racially polarized as any city in America, and this extends to politics.
Until I write my big post on Detroit, I'd definitely recommend a Weekly Standard piece on it from last December titled, "The City Where the Sirens Never Sleep." I'd also point to these great maps from The Map Scroll (via Cartophilia) illustrating how much Motor City has emptied out.