‘Federalism is not just for conservatives.” That was my mantra in 2005, when I represented Angel Raich and Diane Monson before the U.S. Supreme Court. The two California women were challenging the federal ban on medical marijuana as applied to states that authorized its use.

They lost 6-3, and the votes in their favor all came from conservatives: Justices Clarence Thomas and Sandra Day O’Connor and Chief Justice William Rehnquist. Those justices agreed with us—and with the editorial page of this newspaper—that the noncommercial...