Revoke a pardon?

[Sir Mohamed]

Is it possible to revoke a presidential or gubernatorial pardon?

If so, how long? Could Obama for example undo Nixon's pardon? Or could Jimmy Carter have done so?

[MASHED POTATOES. VOTE!]

No. The only way a pardon can be "undone" is rejection by a subject of said pardon, although it happened only once under the Jackson's presidency.

Once a federal pardon is granted, no authority can revoke it, including subsequent Presidents. It's a final and exclusive power vested in the presidency. I'm not sure if it's always a case on the state level, but I wouldn't bet on it.

If it was however, I could imagine Bush, the Congress or the courts revoking certain last-minute pardons issued by Clinton.

[MASHED POTATOES. VOTE!]

Regarding a non-federal level, certain states does not allow governor of grant pardons and commutations at all, with Georgia being a prime example. Others limit these powers to capital cases, like Alabama (the only thing the Governor can do is to commute a death sentence to life in prison). In Texas, the only way the Governor can commute a death sentence is if recommended by the board.

There is limitations on president, except for impeachment cases.