The 80% Romney counties are overwhelmingly rural as a group.
If one goes on population density. the 65 most densely populated counties, independent cities, and the Federal District went heavily for Obama in 2008 -- and McCain broke even in the rest of them. But those densely-populated areas included the Boroughs of New York City, most of northeastern New Jersey, the DC area, Philadelphia, Boston, Chicago, Cleveland, Columbus(OH), Detroit, Milwaukee, Minneapolis-St. Paul, Indianapolis, Baltimore, Richmond, Charlotte, Miami, Tampa, St. Petersburg, Orlando, Denver, Seattle, Portland (OR), and those of the San Francisco Bay Area. Other than those (and they include some rather small cities in Virginia as well as some big cities in States that Obama lost), Obama lost America.
I've seen other people call you out on this before, and I have to ask the same question: why do you use 2008 numbers instead of WAY more relevant 2012 numbers?
