The next president is expected to name at least one new justice to the closely divided court.
Supporters of Proposition 8, which would outlaw the practice, nose ahead in polls.
South Dakota, Colorado, and California weigh measures.
Troy Davis, a Georgia inmate, asked for a new trial after seven witnesses recanted their testimony.
The next president will decide whether the shift away from laissez faire is permanent.
Dick Cheney became the most influential vice president in US history.
The Justice Department says the judiciary does not have the power to release Guantánamo detainees into the US.