Kerry told the Senate Foreign Relations Committee that there is no "unicorn" or "fantasy" alternative if the U.S. rejects the deal, which the administration maintains will keep Iran from getting a nuclear weapon but which many Republicans see as providing Iran a path to a bomb.
But Committee Chairman Bob Corker, a Tennesse Republican, said that the U.S. had been "fleeced" and that Kerry had "turned Iran from being a pariah, to now Congress being a pariah" in the course of making the agreement.
And Florida Sen. Marco Rubio, who is seeking the 2016 Republican presidential nomination, repeatedly warned that the next president could overturn the deal, which isn't a binding treaty.
In the shorter term, Congress could vote against the deal at the end of the 60-day review. President Barack Obama has promised to veto such a vote, but would need to round up a significant number of Democrats in order to sustain the veto.
At Thursday's hearing, it was clear that several Democrats also have concerns about the deal. In one of the hearing's more heated exchanges, New Jersery Democratic Sen. Bob Menendez seemed to give voice to widespread frustration in Congress that members hadn't been more closely consulted during the negotiating process.
When Kerry tried to break in when Menendez, who is extremely skeptical about the deal, was listing his reservations, the senator replied testily: "I have limited time. You've been with the Iranians two years. I have seven minutes."
Virginia Democratic Sen. Tim Kaine, who has yet to say whether he will support the deal, did praise it for rolling back Iran's nuclear program but worried about what would happen when it expires in 15 years.
However, Sen. Babara Boxer, a California Democrat, defended the deal and took issue with Corker's comments.
Still, though no senators appeared to climb down from entrenched positions against and for the deal, the hearing was more notable for substance and detailed examination of the deal's provisions rather than political fireworks.
Kerry said that the U.S. could not simply ignore Iran or walk away from the plan, which calls for new oversight and controls on uranium enrichment in return for an end to economic sanctions.
He laughed briefly in the middle of his testimony before landing on a new Obama administration argument.
"Folks, they already have what they want. They got it 10 years ago or more," Kerry said. "This isn't a question in giving them what they want. I mean, it's a question of how do you hold their program back?"
He told the senators: "Let me underscore the alternative to the deal we have reached is not -- as I've seen some ads on TV suggesting disingenuously -- it isn't a 'better deal,' some sort of unicorn arrangement involving Iran's complete capitulation." He continued, "That is a fantasy, plain and simple, and our intelligence community will tell you that."
Republican Idaho Sen. Jim Risch, however, made clear that Republicans are less than convinced and have every intention of continuing to challenge the deal, suggesting it is the deal's supporters who are out of touch with reality.
"Anyone who believes this is a good deal joins the ranks of the most naïve people on earth," he said.