All the Kings Men (1949)

Directed by Robert Rossen, All the King’s Men is an Academy Award–winning drama inspired by the rise and fall of a populist American politician. The film won three Oscars, including Best Picture and Best Actor, and explores power, corruption, and moral compromise within a democratic system. Often linked to the career of Huey P. Long, it remains one of the most incisive political dramas of classical Hollywood.

Från IMDB:
The rise and fall of a grass-roots rural politician who eventually becomes mired in the sort of political corruption he vehemently railed against on his way to the governor's mansion.