No President has ever made a long-term arms control agreement on his own authority.

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"... this deal may represent the single worst policy outcome of all of Obama’s illegalities.

Mike Ramsey has set forth the case that the Iranian deal is unconstitutional both under the original meaning and under modern law.  Under the original meaning, the Constitution’s provides the way to make major international agreements – through supermajority approval in the Senate, as set forth in Article II, Section 2.  Moreover:

Making major international agreements in the way the text prescribes is not just an eighteenth-century relic; it is the usual course for the United States today (subject to some exceptions noted below).  And the usual course is that if an agreement cannot get two-thirds approval in the Senate, there is no agreement.

What then are the possible justifications for it?  First, the “deal is an executive agreement, done on the President’s independent authority.”  But under the original meaning, there is a strong argument that executive agreements must relate to temporary and minor matters.  Moreover, a similar result obtains under modern law:

the agreements made by prior Presidents under this power have been minor and typically limited to settlements of claims, arrangement of military affairs, diplomatic recognition, and other matters within the President’s military and recognition powers.  No President has ever made a long-term arms control agreement on his own authority.

Second, “The deal is a nonbinding “political commitment” rather than a treaty.  Ramsey claims such nonbinding actions are legitimate under both the original meaning and modern law.  But the Iran deal:

doesn’t look like a nonbinding agreement.  Iran appears to understand it as a binding agreement.  And at least some of its terms appear to (purportedly) constrain U.S. action in the future, beyond the end of President Obama’s term.  It’s likely that a vocal defense of the agreement as nonbinding would substantially undermine the deal.

Finally, Congress will approve the deal.  While the original meaning does not allow a majority of each house of Congress to approve a treaty, in modern times sometimes Congress has done so.  But Congress is very unlikely to approve the deal.

Not only is the deal unconstitutional, its unconstitutionality is essential to it occurring.  If the President had to secure two thirds of the Senate or a majority of both houses, this deal almost certainly would not be approved.

Thus, once again, this President is taking illegal action to make an end run around the Congress to do things that are extremely problematic.  But this time, I fear, the consequences of his action may be worse than in the other cases.  The Iran deal is dangerous."