Justice Antonin Scalia delivered the opinion for a unanimous Court in Smith v. United States, No. 11-8976. You can access the oral argument via this link.  The Supreme Court has ruled that allocating to the defendant the burden of proving withdrawal does not violate the Due Process Clause. Withdrawal is not a frequently used defense. But the Court’s decision has broader application since it presumably sheds light on any affirmative defense.  Unless an affirmative defense negates an element of the crime, the prosecution has no constitutional duty to overcome the defense beyond a reasonable doubt.

See Dixon v. United States, 548 U. S. 1, 6. Withdrawal does not negate an element of the conspiracy crimes charged here, but instead presupposes that the defendant committed the offense. Withdrawal terminates a defendant’s liability for his co-conspirators’ post withdrawal acts, but he remains guilty of conspiracy.

Withdrawal that occurs beyond the statute-of-limitations period provides a complete defense to prosecution, but does not render the underlying conduct noncriminal. Thus, while union of withdrawal with a statute-of-limitations defense can free a defendant of criminal liability nor does it place upon the prosecution a constitutional responsibility to prove that he did not withdraw.