‘We couldn’t have negotiated a better deal.” That is one of the two pillars of the Obama administration’s argument in favor of its nuclear arrangement with Iran, the other being, “there’s no alternative but war.” Those two propositions appear to have won the day—at least with enough Democrats in Congress to prevent a vote disapproving of the Joint Comprehensive Plan of Action. The Iran deal remains deeply unpopular with the American public and with the Republican majority in Congress.

Over the past few months, the two propositions...