The last film that was a full-hearted paen to democracy was "Mr. Smith Goes to Washington" (1939). "Dave" (1993) is a light-hearted fantasy with a democractic ethos at its core. Then there's "All The President's Men" (1976)--can't really describe it as about democracy, but any list of films on American politics has to include it. Some more jaundiced, Tocquevillian looks at American politics are: "Bob Roberts" (1992), "Wag the Dog" (1997), "Primary Colors" (1998), and "The Contender" (2001).
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In print at most public libraries at titles like VideoHound's golden movie retriever, Bowker's complete video directory and the Video source book.