On Monday, I wrote that an Obama-signed executive agreement on Iran would very likely outlast his administration. Regardless of the agreement’s legal status, it would be politically costly for any successor to Obama to renege on any actual deal, assuming that it’s (a) successfully negotiated; and (b) adhered to by Iran as closely as the 2013 interim agreement.

So in the 24 hours since, the political kerfuffle over Sen. Tom Cotton’s letter escalated quickly. I mean, Biden killed a guy with a trident put out a pretty nasty statement averring, “In 36 years in the United States Senate, I cannot recall another instance in which Senators wrote directly to advise another country — much less a longtime foreign adversary — that the President does not have the constitutional authority to reach a meaningful understanding with them.” President Obama didn’t go that far, but he did pick up on the “Star Trek VI”-like oddball coalition of hard-liners.

Cotton (R-Ark.) fired back Tuesday morning, blasting Biden’s foreign policy record and demanding that the administration put any agreement with Iran up for Senate approval. What’s interesting, however, is that Cotton’s MSNBC hit didn’t bring up Iranian Foreign Minister Mohammad Javad Zarif’s open response to Cotton’s letter, which rocketed around Twitter on Monday night:

— Javad Zarif (@JZarif) March 10, 2015

I don’t agree with all of Zarif’s response, but he did bring up two pretty compelling reasons why the deal might stick despite opposition that I did not address in my previous post:

[Zarif] emphasized that if the current negotiation with P5+1 result in a Joint Comprehensive Plan of Action, it will not be a bilateral agreement between Iran and the US, but rather one that will be concluded with the participation of five other countries, including all permanent members of the Security Council, and will also be endorsed by a Security Council resolution.
Zarif expressed the hope that his comments “may enrich the knowledge of the authors to recognize that according to international law, Congress may not modify the terms of the agreement at any time as they claim, and if Congress adopts any measure to impede its implementation, it will have committed a material breach of US obligations.

This is an important point that often gets lost in the partisan rancor. The Iran negotiations are not a bilateral arrangement between Iran and the United States, but a P5+1 negotiation with Iran. If a deal is reached, it’s a deal that has the support of all the permanent members of the U.N. Security Council plus Germany.

Now I doubt that Tom Cotton et al would weep much if, in undermining an executive agreement, they would tick off, say, Russia or China. But our NATO allies in Europe are another question entirely. Does the next president want one of his/her first actions to be revoking a deal negotiated in part by America’s closest allies? Methinks not.

Zarif brought up another reason why he thinks GOP senators are bluffing:

The Foreign Minister also informed the authors that majority of US international agreements in recent decades are in fact what the signatories describe as “mere executive agreements” and not treaties ratified by the Senate.
He reminded them that “their letter in fact undermines the credibility of thousands of such mere executive agreements that have been or will be entered into by the US with various other governments.

I’m less convinced that this would act as a deterrent to Republican members of Congress, who do have some legitimate gripes with this administration when it comes to executive power. But it would act as a deterrent for the next president, and that’s the actor that really matters in Iran’s calculations. The next president is going to ink a lot of executive agreements, as well, and would not want the credibility of this legal instrument in general to be sabotaged.

Zarif’s response reinforces my conclusion from Monday: This open letter will have minimal impact on the Iran negotiations, and any executive agreement signed by Obama will likely outlive his administration. And it’s that unacknowledged fact that so enrages Republican senators.