Prisoners should be allowed to vote because voting is a right of citizenship. Citizenship is not dependent on virtue and it cannot be revoked as a punishment for crime. The Supreme Court decided this principle in Trop v. Dulles (1958). Obviously, a prisoner forfeits some of the rights of citizenship during the period of his or her incarceration, the most obvious of which is liberty. This, however, is an intrinsic part of the sentence. It is necessary for the purposes of both punishment and rehabilitation that the prisoner should be incarcerated. It is not necessary for either purpose that the prisoner should lose the right to vote. Indeed, participation in the democratic process is a useful part of the prisoner's rehabilitation, making it clear that the prisoner is still a part of society. Moreover, the ability of government to confiscate the rights of citizens should be curtailed as much as possible.
The United States of America incarcerates an exceptionally high percentage of its population, many of them for drug-related crimes and minor offenses which would not even be criminal in other jurisdictions. If these people are treated as "civically dead," politicians will not be motivated to ensure that conditions in prison are relatively humane, peaceful and salubrious. The participation of some two million prisoners in the process of democracy will help to compel the political establishment to address issues such as prison abuse and violence, in addition to making the transition upon release easier for inmates who have remained politically engaged citizens.