GALLUP: Democratic Enthusaism down sharply from 2004, 2008

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This is a generalization, with all the weaknesses generalizations are prone to, of course.  But I think it's easier for voters to be enthusiastic when their guy is either running against an actual incumbent (2004) of against the outgoing incumbent's party.  Defending an incumbent, especially when the incumbent's own side has such misgivings about or is so disappointed in them, makes everything harder.  Add to that the crummy prolonged unemployment numbers, and the bottoming-out D enthusiasm doesn't surprise me at all.