City Liberals, Rural Republicans

In the forums Elemenope made this observation:

Cities pretty much everywhere in the US are liberal. The rural areas…pretty much everywhere in the US are conservative. For a fun time, overlay a population by county map with county returns for any presidential election over the last 20 years. Almost *completely* without exception, the urban counties go for the democrat, the rural counties go for the republican.

Any theories as to why this is the case?

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