From Prof. Curtis Bradley . . .
As for the merits of the Medellin case, one point to keep in mind is that, in order for the petitioner to obtain relief, he must find a basis for preempting otherwise applicable state laws concerning the finality of convictions and sentences. The question of when state laws are preempted in U.S. courts, however, is purely a question of domestic law, not a question of international law. As the Supremacy Clause of our Constitution makes clear, there are only three types of law that can preempt state law in U.S. courts - the Constitution, federal statutes, and treaties. International judgments do not by themselves preempt state law.
It is still possible for Medellin to argue, as I noted in the debate, that the underlying treaty interpreted by the ICJ (the Vienna Convention on Consular Relations) has preemptive effect, and that U.S. courts should give weight to the ICJ's views about the meaning of the treaty. The Supreme Court in Breard stated that the ICJ's views about the meaning of the Vienna Convention are entitled to "respectful consideration," and I think that is about right. Ultimately, however, U.S. courts would need to make their own decision about the treaty's meaning, just as they make their own decision about the meaning of other laws that they apply.
In addition to fitting with the text of the Supremacy Clause, focusing on judicial enforcement of the underlying treaty rather than the ICJ judgment preserves a greater role for politically accountable actors (Congress and the Executive Branch) in deciding how to implement international decisions, which is consistent with U.S. democratic values. It also ensures that U.S. courts and the political branches have some ability to avoid implementing patently unreasonable or ultra vires rulings by international tribunals. Finally, it avoids the Article III and federalism concerns associated with allowing non-U.S. judges to directly override U.S. laws.