In which film did Peter Sellers play a simple-minded gardener who ended up as a presidential advisor?

Being There is a 1979 American comedy-drama film directed by Hal Ashby. Its screenplay was adapted by Jerzy Kosiński and the uncredited Robert C. Jones from the 1970 novel by Kosiński. The film stars Peter Sellers, Shirley MacLaine, Melvyn Douglas, Richard A. Dysart, Jack Warden, and Richard Basehart.

Douglas won the Academy Award for Best Supporting Actor and Sellers was nominated for Best Actor. The screenplay won the British Academy Film Award for Best Screenplay and the Writers Guild of America Award for Best Comedy Adapted from Another Medium. It was also nominated for the Golden Globe Award for Best Screenplay.

A simple-minded gardener named Chance has spent all his life in the Washington D.C. house of an old man. When the man dies, Chance is put out on the street with no knowledge of the world except what he has learned from television. After a run in with a limousine, he ends up a guest of a woman (Eve) and her husband Ben, an influential but sickly businessman. Now called Chauncey Gardner, Chance becomes friend and confidante to Ben, and an unlikely political insider.