From a Bureau of Labor Statistics news release:
In 2011, the number of workers belonging to a union in Michigan was 671,000, the U.S. Bureau of Labor Statistics reported [on Monday, April 9, 2012]. Regional Commissioner Charlene Peiffer noted that union members accounted for 17.5 percent of wage and salary workers in Michigan in 2011 compared to 16.5 percent a year earlier. At its peak in 1989, the first year state data were available, Michigan’s union membership rate was 26.0 percent. (See chart 1 and table A.)
The full news release has lots of detailed data, but the map below is the keeper.