From Special Counsel Jack Smith's filing in U.S. v. Trump:
It has long been the position of the Department of Justice that the United States Constitution forbids the federal indictment and subsequent criminal prosecution of a sitting President. But the Department and the country have never faced the circumstance here, where a federal indictment against a private citizen has been returned by a grand jury and a criminal prosecution is already underway when the defendant is elected President.
Confronted with this unprecedented situation, the Special Counsel's Office consulted with the Department's Office of Legal Counsel (OLC), whose interpretation of constitutional questions such as those raised here is binding on Department prosecutors. After careful consideration, the Department has determined that OLC's prior opinions concerning the Constitution's prohibition on federal indictment and prosecution of a sitting President apply to this situation and that as a result this prosecution must be dismissed before the defendant is inaugurated.
That prohibition is categorical and does not turn on the gravity of the crimes charged, the strength of the Government's proof, or the merits of the prosecution, which the Government stands fully behind. Based on the Department's interpretation of the Constitution, the Government moves for dismissal without prejudice of the superseding indictment under Federal Rule of Criminal Procedure 48(a)….
