Chart borrowed from David Park relates the share of the vote going to Democrats to the share of House seats controlled by Democrats. You can see that starting in 1994 we entered an era when Democrats have consistently underperformed their vote share. If the current Democratic majority can stay in place past the 2010 census, one assumes that will change. Still, it should always be remembered -- but especially in these days of heady optimism -- that the structure of American political institutions provides a substantial bias in favor of conservatism and makes it difficult for small progressive majorities to accomplish very much.

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