Missouri v. Holland
Yesterday the Supreme Court granted certiorari in Bond v. United States, which gives the Justices an opportunity to revisit Missouri v. Holland, which said (in 1920) that treaties could give Congress powers beyond those granted in Article I, Section 8. (Or, put another way, that measures taken to execute a treaty under the Necessary and Proper Clause are constitutional though they would not be otherwise.)
Missouri v. Holland has always struck me as a strange decision. Part of that is because it was written by Holmes in his usual cryptic way. Part of it is that the opinion said that there are some justiciable limits to the treaty power (or treaty augmentation power), but didn’t say much about what those are. Now in an era where the enumerated powers of Congress were read more narrowly, I can understand why the Court might have wanted to open the door for other sources of federal authority. Today, though, this notion seems either unnecessary or wrongheaded.
One way to solve this dilemma would be to reintroduce a distinction between treaties under Article II and executive-congressional agreements under Article I. (Right now, these are treated as functionally equivalent.) In other words, maybe a treaty that is ratified by a supermajority of the Senate should be able to increase the powers of Congress, but an executive-congressional agreement ratified by majorities in each House just like a bill should not. This would allow the Court to reaffirm Missouri while limiting its scope, as most modern international agreements are not ratified as Article II treaties.
For more on this, I would encourage you to look at the ongoing exchange over on Volokh between Rick Pildes and Nick Rosencranz.