Even though it's a longshot that the United States would elect a prisoner, it has actually been done more than once! In 2004, Lyndon LaRouche, an 81 year old Democrat ran for President for the eighth time. On his fifth try in 1992 he ran from a prison cell where he was serving for tax evasion, mail fraud, and conspiracy.
In 2004, Leonard Peltier, an American Indian activist from the Peace and Freedom party ran for President from prison, as well. His crime was killing two FBI agents in 1975 in South Dakota. Apparently he’s been trying to get a parole hearing since 1986.
Though people were disgusted by Peltier and the Peace and Freedom party for letting him run, they had every right to put him on the ballot. So, next time you vote, make sure you know who everyone is first!