From the Congressional Research Service:
Can the President issue a so-called “prospective pardon”?
With respect to crimes committed prior to the issuance of the pardon, it appears the President can issue a pardon before any criminal proceeding against the pardon recipient has been initiated. In the1866 case ex parte Garland, the Supreme Court announced that the pardon power “extends to every offence known to the law, and may be exercised at any time after its commission, either before legal proceedings are taken or during their pendency, or after conviction and judgment.” Put another way, for the President to issue a pardon, the crime must have already been committed, but the President need not wait for an indictment or other information before granting the pardon. Take, for example, President Gerald Ford’s pardon for former President Richard Nixon, which granted Nixon “a full, free, and absolute pardon . . . for all offenses against the United States which he . . . has committed or may have committed or taken part in during the period from January 20, 1969 through August 9, 1974.” Although no indictment had been brought against Nixon, his pardon shielded him from any future federal prosecution based upon any criminal acts he may have committed during the time period stated. A 1995 memo from the Department of Justice Office of Legal Counsel (OLC) confirms this understanding by the executive branch, noting that “throughout the Nation’s history, Presidents have asserted the power to issue pardons prior to conviction, and the consistent view of the Attorneys General has been that such pardons have as full an effect as pardons issued after conviction.”
Can the President pardon himself?
The text of the Pardon Clause does not speak to whether the President can pardon himself. The Framers did not debate this question at the Convention, and it unclear whether they considered whether the pardon power could be applied in this manner. No President has attempted to pardon himself. In a memorandum issued several days before President Nixon resigned, the OLC concluded that the President could not issue a self-pardon, positing that “under the fundamental rule that no one may be a judge in his own case, the President cannot pardon himself.” This general legal assertion closely echoes Federalist No. 10, in which James Madison declared that “[n]o man is allowed to be a judge in his own cause, because his interest would certainly bias his judgment, and, not improbably, corrupt his integrity.” However, Madison’s comments were not made in the context of pardons, and appear to be a general assertion about the rule of law, rather than an opinion on the constitutionality of self-pardons. While some legal scholars have contended that the President cannot pardon himself (see, e.g., here and here), others contend (see, e.g., here) that a good-faith argument, at least, could be made to support a self-pardon’s constitutional validity given the lack of textual restrictions against self-pardons within the Constitution. Accordingly, this is an unsettled constitutional question, unlikely to be resolved unless a President acts to pardon himself for a criminal offense