Estimating the Union Vote Premium

I'm familiar of course with the fact that union households are more inclined to vote Democratic than are members of non-union households, but since union membership isn't distributed randomly amongst the population it's a bit difficult to know exactly what to make of that. Nate Silver, however, took National Election Survey data to do a logistic regression analysis to try to isolate the impact of specific demographic characteristics on the 2008 presidential vote:

Union status, roughly speaking, is less important than race or religion but more important than the rest.