The film Abe Lincoln in Illinois (1940) has perhaps the best recreation of the Lincoln-Douglas debates ever put on film. The debate portrayed has remarks culled from all the debates, is an excellent recreation of the main arguments made by each of the men, and is evocative of their speaking styles.
Ironically neither of the actors portraying Lincoln and Douglas were Americans. The actor portraying Douglas was Gene Lockhart, a Canadian. If his voice sounds vaguely familiar to you, it is probably because you recall him as the judge in Miracle on 34th Street. His daughter June Lockhart, of Lassie and Lost in Space fame, carried on the thespian tradition of the family.
Lincoln was portrayed by Raymond Massey, also a Canadian. Massey was one of the great actors of his day and bore a strong physical resemblance to Lincoln. Massey served in the Canadian Army in both World War I, where he saw combat on the Western Front as an artillery officer, and World War II, becoming a naturalized American citizen after World War II. Like Lincoln he was a Republican and made a TV ad for Goldwater in the 1964 campaign.