U.S. Constitution, Article II, Section 2: "The President {irrelevant material omitted} shall have power to grant reprieves and pardons for offenses against the United States, except in cases of impeachment." Anything that would limit his power to pardon himself, before or after an offense or a charge, would require a Constitutional amendment...unless, as I read it, the charge is brought under Congress's impeachment powers. A bill was brought before Congress back in the 1930s that would require a conviction before a pardon could be granted, but it died.