There has been a great deal of commentary about the propriety and even the constitutionality of a group of senators sending a letter to the Iranian leadership laying out the lack of support to the proposed deal regarding Iran's nuclear program. Opposition to the deal is strong in broad segments of the Congress, and the letter warns that any agreement that is reached only with the Executive Branch is subject to being overturned by a future presidential administration or being altered by Congressional legislation.
I want here instead to focus on an ethical issue: whether or not this is part of any full disclosure that must be made to Iran.
Executive agreements—that is, arrangements made with other states that are not treaties subject to Senate ratification—exist in a gray area. A treaty that has been ratified by the Senate is, according to Article VI of the U.S. Constitution, equal in weight to the Constitution itself in terms of its binding effect upon the government of the United States. Thus, per the ratification of the UN Charter, for instance, the United States is obligated to carry out resolutions of the UN Security Council (of course, one of the ways President Truman was able to get the Charter ratified was to assure the Senate that America's veto power meant that it could never be compelled to do something).
Executive agreements are binding on the executive branch; that is, if the President issues a directive guiding policy, the departments and agencies will follow it. Even though the United States has not ratified the Law on the Sea (UNCLOS) Treaty, the agencies of the U.S. government follow its directives by virtue of executive orders decreeing U.S. compliance with many of its provisions. The courts have understood these agreements to have power to bind the U.S. government and to be honored—but that is based on a critical assumption: that the terms of that agreement are not subsequently impacted by Congressional legislation. Two big "ifs": if the Republicans retain control of Congress, and a Republican is elected in 2016, it might be possible that the terms of any agreement reached by the Obama administration could be overturned by Congressional action that would be signed into law by another President.
There is also the fact that executive understandings are not enforceable if the United States does not wish to act. Case in point: the Budapest Memorandum of 1994, which in theory commits the United States to secure the territorial integrity of Ukraine in return for Ukraine having given up control of its share of the Soviet nuclear arsenal. When it became inconvenient to enforce its terms after the Russian annexation of Crimea in 2014, there was no mechanism to force the United States to assist Ukraine. (The fate of Ukraine and the weakness of the Budapest guarantees, by the way, was not lost on the Iranian leadership, who sees that and what happened to Muammar Gaddhafi of Libya as proof that promises do not carry a great deal of weight).
The letter certainly undermines the negotiations now underway in Vienna and are an embarrassment to the Obama administration. But the situation it describes is a very real one. The Islamic Republic may very well decide that even with the flaws of concluding an executive agreement with the United States, the gains outweigh the risks. But one can argue that one party to any set of talks ought to fully understand all the nuances of the political system of its interlocutor. The reality is that the Iran deal will rest on the commitment of the Obama administration to carry it out for the remainder of its term of office. It may solidify the deal in such a way as the costs for overturning it by a successor administration become too high, but that is not the same guarantee as ratification. Iran might get a better deal by accepting an executive agreement than it would if the deal had to pass the Senate, but it should be aware of what those risks are. Those issues, no matter the ways in which they were broadcast, are now fully out in the open.