“We are all united against a nuclear Iran,” he quipped, as he cast a glance at Mr. Samore.

Mr. Samore, who now runs the Belfer Center for Science and International Affairs at Harvard, initially said that the chances of a successful negotiation were dim. But after the framework of an accord was announced in Lausanne, Switzerland, in April, he praised it as a good first step.

Mark D. Wallace, the chief executive of the group and a diplomat in the George W. Bush administration, said that the organization’s members had sought to keep an open mind. But after the final terms became clear, “The opposition was nearly unanimous,” he said.

With that move, it was clear that Mr. Samore needed to move on.

“We had an honest discussion that I wouldn’t be able to continue to serve as president if UANI was going to come out against the agreement, since I support it,” Mr. Samore said.

“Nonetheless, I support the work that UANI has done in the past to strengthen sanctions, and I think they will have a role to play in the future to maintain nonnuclear sanctions if the deal goes forward,” he said. (He will continue to serve on the group’s advisory board.)

Though he backs the accord as the most that can be achieved diplomatically, Mr. Samore is skeptical that the agreement will open a new chapter in American-Iranian relations.

“The best you can achieve with diplomacy is delay in the hope that at some point a new Iranian government emerges that is not committed to developing nuclear weapons,” he said.