The Constitution grants the president the power to pardon "offenses against the United States". An offense that violates state law, but not federal law, is an offense against that state rather than an offense against the United States; however, the Supreme Court has never ruled on this matter or in the President's power to grant a habeas corpus petition for a state offense where it has been denied by a federal court.

So it's never been tested in court, the constitution doesn't very explicitly deny the power... Not great odds with the current supreme court