American politics turn on a now familiar set of categories: red states vs. blue states, rich states vs. poor states, Frostbelt vs. Sunbelt. But these generalizations mask deeper, less visible fissures in our political geography. We have written a great deal about the role of density in metropolitan voting patterns, highlighting the remarkably consistent and robust political red-to-blue tipping point that occurs when a metro reaches a density of roughly 800 residents per square mile. I took a deeper look at our emerging political geography in a recent feature for Politico magazine, where I argued that the suburbs have become...