The Real Problem With Medellin
"Foreign Affairs Exceptionalism" is a description of the structural transfer of power from the states and Congress to the federal government and the Executive in foreign affairs. The Court grants deference to the Executive in its interpretation of treaties and in its interpretation of some areas of the Constitution; states rights are not protected as carefully as they are domestically.
This exceptionalism can be said to allow the federal government, and even the President to delegate authority internationally. These treaty interpretation regimes have been around since the 1794 Jay Treaty. If the importance of the non-delegation doctrine is exaggerated domestically (Adrian Vermeule & Eric Posner, Interring the Nondelegation Doctrine, 69 U. Chi. L. Rev. 1721 (2002)), there is a strong argument that any international constraint on the federal government's power to delegate authority to more easily enter international agreements is also exaggerated.
The problem is that this is not the argument being made by petitioners in Medellin. Their argument is not only that Congress can delegate this power, but that they did delegate it. This, however, does not work. At least not without overruling Marbury v. Madison.
The Court, per Marbury, must pass some form of judgment on the ICJ's interpretation of the treaty. They may, as they did in Chevron, not pass independent judgment, but only police the boundaries of the delegation to ensure that the tribunal is acting within the scope of the delegation. This means that a reasonable interpretation fo the treaty by the ICJ would "bind" the Supreme Court. They would have to completely defer to the interpretation within that scope.
But this, I think, is the best petitioners can do. And since the Vienna Convention not only doesn't include any preemptive power over adequate state grounds in the delegation, but expressly states that adjudication of the convention's rights are to follow the rules of the forum states' courts, the decision by the ICJ to preempt an adequate state procedural default rule is outside the scope of the delegation, and therefore unreasonable.
Finding otherwise would be a new kind of exceptionalism that not only transfers power within the federal government, but transfers power away from the federal government, to the ICJ. Furthermore, if the federal government were unable to, either by failing to include a power in the delegation, or even by expressly excluding it, constrain the scope of its international obligations, every delegation would be a blank check for the foreign tribunal, changeable only by exit from the regime. This is obviously bad--not only for those of us concerned primarily with the preserving the democratic structure of the Constitution, but for the international rule of law as well.