Chuck Schumer’s decision to oppose President Obama’s Iran nuclear deal may not defeat the accord, but it certainly does showcase its flagging political support. Mr. Schumer is a party stalwart who wants to succeed Harry Reid as Senate leader, and his defection suggests that the deal will be opposed by at least a bipartisan majority in both houses of Congress.

Think about how extraordinary that would be. Major foreign policy initiatives are often controversial, but they typically garner at least majority support. The resolutions...