There is no possibility of reforming the coalition to negotiate a "better deal"

At this point, a “better deal” is an illusion. One can always imagine a better deal. But if the U.S. Congress rejects this agreement and proposes sending Secretary of State John Kerry back to the negotiating table, Kerry will most likely find no one else there. Partners who have negotiated and compromised over 20 months to achieve this accord will conclude that the U.S. government is incapable of making agreements. The international coalition will splinter and the sanctions regime will collapse, with Russia and China leading the way, but with France and Germany not far behind. The United States will have demonstrated that D.C. is in fact an acronym for Dysfunctional Capital. In comparison, Iran will appear to be the adult in this paring.