So timing would be everything. Will we ever know for sure whether a self-pardon is constitutionally permissible? Unlikely.
First, a president would have to pardon himself. The political cost of such an action would be very high. Second, a prosecutor would then have to try to prosecute the former president. Third, the former president would have to raise the pardon as a defense.
Finally, the courts would have to decide whether, under our system of separation of powers, the courts have jurisdiction to review a presidential self-pardon. This last contingency is interesting, because as Justice Oliver Wendell Holmes reminded us, the power to pardon is not a mere “private act of grace,” but rather an important part of our “constitutional scheme,” of checks and balances and separation of powers.