"What do you get by not talking? You get closer to a bomb," Kerry warned the lawmakers.
Kerry's tone in the nearly two-hour hearing was alternately defiant and pleading as he lobbied Congress to scrap plans for more penalties on Iran during the first six-month phase of the agreement, which was brokered last month in Geneva by the United States and fellow U.N. Security Council permanent members China, France, Russia and the United Kingdom, plus Germany.
Kerry said that imposing deeper sanctions now would send a signal to those partners that the United States wouldn't uphold its end of the deal. And, he said, such action would embolden Iranian hardliners who oppose relatively moderate President Hasan Rouhani's overtures to the West, and only make the regime more intent on flouting international inspectors and going after a bomb.
On the other hand, Kerry suggested, if all went well there was an "outside chance" that a final, comprehensive nuclear deal could be reached even before the six-month mark.
"Let me be very clear: This is a very delicate diplomatic moment, and we have a chance to address peacefully one of the most pressing national security concerns that the world faces today," Kerry said.
It's uncertain whether Kerry was able to sway the legislators. A bipartisan bloc has drafted sanctions legislation to attach to the annual defense authorization bill, which could be put to a vote before Congress breaks for the holidays.
At the hearing, both Republicans and Democrats voiced a litany of similar concerns: that the deal jeopardizes Iran's archenemy, Israel; that there's no basis for trusting a pariah state that's deemed a sponsor of terrorism; and that the deal allows Iran to keep enriching uranium.
Rep. Ileana Ros-Lehtinen, R-Fla., went further than most of her fellow committee members, declaring the Geneva agreement a "bad deal."
In response to lawmakers' charges that the Obama administration was naive in trusting Iran, Kerry said the goal was to "test, but verify," a play on Ronald Reagan's mantra of "trust, but verify" during talks with the former Soviet Union.