Mr. Smith Goes to Washington

One of my favorites! From Wikipedia:
Mr. Smith Goes to Washington is a 1939 drama directed by Frank Capra about American government. Nominated for 11 Academy Awards upon its release (it won Best Screenplay), the film made James Stewart a major movie star. It also starred Jean Arthur and Claude Rains, as well as a bevy of well-known supporting actors, among them Edward Arnold, Guy Kibbee, Charles Lane, and Thomas Mitchell. It was written by Lewis R. Foster and Sidney Buchman, based on Foster's novel, The Gentleman from Montana.

The governor of an unnamed state, Hubert "Happy" Hopper (Guy Kibbee), has to pick a replacement to fill the unexpired term of a deceased Senator. His corrupt political boss, Jim Taylor (Edward Arnold), pressures Hopper to choose his handpicked stooge, while popular committees want a reformer. The governor's children want him to select Jefferson Smith (James Stewart), the head of the Boy Rangers.