A Face in the Crowd is a good one from Elia Kazan. Though not 100% about politics, it is about the cult of personality and how it bleeds over into everything including politics.
Also known as "the Glenn Beck story."
I'd recommend. "advise and consent," a film about the DC confirmation process.
http://www.imdb.com/title/tt0055728/"The Best Man," a movie about a contested political convention is another good one one:
http://www.imdb.com/title/tt0057883/?ref_=tt_rec_tt
