It seems that American political campaigns start earlier and earlier with each political cycle.
However, if you thought the 2008 presidential race started early, check out this article from the April 16, 1908 Cedar Rapids Evening Gazette (Cedar Rapids, IA) titled, "'Count' Opens Campaign for Presidency in Year 2008." An excerpt along with the piece in its entirety appear below.
Charles Vaden Barton, "the count," one of the choicest cranks that ever infested the capital, has arrived from Seattle to open his campaign to elect himself president in 2008. He announces that he is the John the Baptist of the millennium, and as he has special arrangements by which he beats the undertakers and cannot die, he can start his presidential campaign a long time ahead and work up sentiment gradually. So he is starting 100 years ahead, and expects that by the time he is elected the millennium will begin coincident with his inauguration.
Lyndon B. Johnson on 2063 A.D. (1963)
Hubert H. Humphrey's Future (1967)
Hubert H. Humphrey's Year 2000 (1967)
Governor Knight and the Videophone (Oakland Tribune, 1955)
Edmund G. Brown's Californifuture (1963)
Television: Medium of the Future (1949)
Fruition of Ideals of Democracy (1923)