In 1974, President Richard M. Nixon’s own Justice Department’s Office of Legal Counsel issued an opinion stating that Nixon could not pardon himself, based upon “the fundamental rule that no one may be a judge in his own case.” Likely for that reason, Nixon never pardoned himself.
If Trump were to try to take that step, presumably under the theory that Democrats will retaliate against him, Biden should first refer the question to the Office of Legal Counsel. If the OLC in 2021 concurs with the precedent of that office in 1974 — which is highly likely — this legal opinion would constitute a second piece of guidance shoring up the position that self-pardons are inherently unconstitutional. It would provide a basis for President Biden to then issue an executive order nullifying Trump’s action.
That action is especially important because Biden seems to be disinclined to have his Justice Department prosecute Trump in the interest of moving on. This means there may never be any “case or controversy” involving the self-pardon issue; the question would arise only if federal criminal charges were filed against the former president. Thus, it’s quite possible the matter will end there and never get to court. If it did, the Justice Department’s consistent interpretation of limited presidential power would be influential — another reason for the new administration to weigh in.