Is the deal announced yesterday with Iran unconstitutional?

In a word: probably.

Here is my assessment. To begin, the Constitution’s text provides the way to make major international agreements – through supermajority approval in the Senate, as set forth in Article II, Section 2: “The President … shall have power, by and with the Advice and Consent of the Senate, to make Treaties, provided two thirds of the Senators present concur.” This is, moreover, the way the framers and ratifiers understood it: every discussion of international agreement-making in the founding era assumed it would take place through the Senate as Article II, Section 2 describes. Some argued that this was not enough protection against harmful treaties, and wanted a higher bar – three-quarters of the Senators present, or two-thirds of all Senators, not just of those present. No one contemplated that treaties could be made in an alternate, less demanding way.