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Whether Americans have “sorted” into politically like-minded counties and to what extent is hotly debated by academic and journalists. This paper examines whether or not geographic sorting has occurred and why it has occurred using a novel, dynamic analysis. Our findings indicate that geographic sorting is on the rise, but that it is a very recent phenomenon. In the 1970s and 1980s, counties tended to become more competitive, but by 1996 a pattern of partisan sorting had emerged and continued through the present. Results suggest this pattern is driven by Southern re-alignment and voting behavior in partisan stronghold counties. Lastly, we find evidence that migration can drive partisan sorting, but only accounts for a small portion of the change.