From the comments on an earlier post, here's DC trade lawyer Ted Posner:
This discussion of how the United States will approve the newly concluded TFA is quite interesting. From my quick perusal of the agreement, I expect that the USG view that no congressional action is required is based on the determination that no U.S. law needs to be amended to give effect to the agreement, as Simon suggested. In other words, the USG appears to be treating this as an agreement that the President is constitutionally authorized to enter into under his sole executive power without congressional action. That certainly is not unprecedented. The United States enters into plenty of "sole executive agreements." For example, "trade and investment framework agreements" (TIFAs) are conlcuded without express congressional approval. I believe the Anti-Counterfeiting Trade Agreement was not submitted to the U.S. Congress for approval. Even the GATT 1947 did not receive express U.S. congressional approval. (It was given effect through presidential proclamation pursuant to authority given the President under the 1934 Reciprocal Trade Act.) What's odd here is that the U.S. Congress did expressly approve the WTO Agreement (see Uruguay Agreements Act sec. 101(a)), and the new TFA would appear to be an amendment to the WTO Agreement. So query whether, as a matter of U.S. constitutional law, where an international agreement was treated as a "congressional-executive" agreement when it was approved, that agreement can be amended through an agreement that is treated as a "sole executive" agreement. I am not aware of any precedent for this. I would be curious whether anyone is. As I am sure many will recall, at the time the United States Congress approved the Uruguay Round agreements and the NAFTA, there was much debate among constitutional law scholars (notably, Prof. Lawrence Tribe and Prof. Bruce Ackerman) about whether the United States could treat such agreements as "congressional-executive agreements" (as opposed to "treaties" which, under Art. II of the Constitution would require approval by a two-thirds majority in the Senate). The prevailing view appears to be that as a matter of constitutional law the United States does not need to treat such agreements as "treaties" (even if they are considered to be treaties as a matter of international law). But now we are going a step beyond that. Now the question is whether a "sole executive" agreement can amend an agreement that was approved as a "congressional-executive" agreement. I will be eager to hear what others have to say on the topic.
My guess is that we will probably muddle along for quite a while with only limited clarity about what is permitted here. Only if some confrontation erupts at some point between the legislative and executive branches, in the trade field or elsewhere, will everyone have to sort things out. As long as the executive confers informally with the legislature first, and doesn't push too far, it will maintain the ability to expand international law on its own through sole executive agreements, including as amendments to congressional-executive agreements.