The new agreement between Iran and the six powers (including the United States) on Iran's nuclear weapons development is called a "Joint Plan of Action." For constitutional purposes, what is that?
I assume it's not a treaty, because there has been no talk of submitting it to the Senate (and the Senate might well not approve it. And I assume there is no statutory authorization for it, so it is not an "ex-ante" congressional-executive agreement (even if such a thing were constitutional for this sort of a deal).
Is it a sole executive agreement (that is, a international agreement done on behalf of the United States on the President's independent authority)? I have argued (The Constitution's Text in Foreign Affairs, Ch. 9) that under that Constitution's original meaning the President does have some power to make sole executive agreements, especially including temporary ones (the "Joint Plan" has a duration of six months). But that power is narrow, and it is not clear it would encompass promising (as the "Joint Plan" does, p. 3) that "The U.S. Administration, acting consistent with the respective roles of the President and the Congress, will refrain from imposing new nuclear-related sanctions [on Iran]," even on a temporary basis. (A stronger argument could be made that the President does have power to agree, at least on a temporary basis, to "suspend" existing sanctions -- as other parts of the Joint Plan promise -- assuming he has statutory authority to do so under existing law).
At Opinio Juris, Duncan Hollis has what appears to me to be the right answer: the "Joint Plan" is not a binding agreement under international law. As he points out, "the operative paragraphs maintain an emphasis on avoiding language of legal intent," including by using words such as "voluntary measures" and "would undertake" (rather than "shall undertake") -- plus the document is unsigned.
Does that make it constitutional? Here's Professor Hollis' assessment:
For the United States, though, I think the third, and most significant, implication of this deal taking a political form is the fact that the Obama Administration doesn’t have to get the Senate or the Congress as a whole to approve it. Legally binding treaties and international agreements require the conclusion of specific domestic approval procedures. The Constitution contemplates the Senate giving advice and consent by a 2/3rd majority to Treaties (and most arms control agreements are done as Treaties). Modern practice meanwhile more regularly favors ‘congressional-executive’ agreements where Congress approves of the conclusion of the agreement before or after the deal is done. In other cases, the President may invoke his sole executive powers to authorize the conclusion of a deal by himself. But, when it comes to political commitments, there are no constitutional precedents requiring that Congress as a whole or the Senate authorize the commitment’s conclusion. Now, together with Josh Newcomer, I’ve argued previously that this status quo is constitutionally problematic where political commitments can function in much the same way as treaties. I fear political commitments may function as a loop-hole for the Executive to do deals that he could not do if he had to go to Congress or the Senate. I’m not sure that this is such a case, but it’s certainly worth thinking about the consequences of having the United States pursue this major foreign policy shift where the U.S. legislature has so little say in the matter (at least until such time as any deal requires changes to U.S. law itself).
My view is this: Hollis and Newcomber may be right to be concerned as a policy matter, and their outstanding article is well worth reading, but "political commitments" by the President are constitutional. The President has diplomatic power as part of his "executive Power" in Article II, Section 1. So he can tell other governments what he will do or not do (of course, he can change his mind later). That communication can take the form of an undertaking to do (or not do) something if the other side does (or doesn't do) something in return. This is just a diplomatic arrangement (a "Gentlemen's agreement," as it was called in the early twentieth century). Because the President is (by definition) not binding himself or the United States under international law, the Constitution's limits on international agreement-making aren't triggered.
I think this has to be the rule as a practical matter as well, because the President and Iran could accomplish the same thing through reciprocal policy announcements. That is, the President could announce that he will, for example, suspend sanctions so long as Iran takes certain steps, and Iran could announce that it will take the specified steps so long as sanctions are lifted. Nothing need be committed to paper, or even agreed face-to-face. Surely this is within the President's diplomatic power (again, assuming he has statutory authority to lift the sanctions). True, it is a "major foreign policy shift," but the executive diplomatic power allows the President to make such shifts. And the fact that, in this case, the reciprocal policies are worked out in advance and written down should not make a constitutional difference.
This means the "Joint Plan" is constitutional. But it also means Iran (or the United States) can depart from it without violating any legal commitment.