The hearing marked a new phase of a bruising struggle that will lead to what will arguably be the biggest foreign policy vote in more than a decade.
The deal will take effect unless Congress blocks it, and Republicans in control of the House and Senate have made clear they intend to try to do so in September.
Obama has vowed to veto any such bill. That would lead to a vote to override his veto. To foil any such move, the administration would need either 34 votes in the Senate or 146 in the House — a tally the White House is widely expected to achieve.
Democrats emerging from Wednesday's closed-door briefing sounded optimistic the administration would be successful.
"I'd be shocked if there's more than a handful of Republican 'yes' votes, if there are any at all," said Sen. Chris Murphy, D-Connecticut. "But I've talked to most all my colleagues on the Democratic side...I think the administration is, one by one, picking off some of the most important lingering questions from Democrats."
Murphy, another member of the committee, said he still has questions about whether the inspection protocol will be as rigorous as the administration claims, but that if it is, he'll support the deal.
Democrats and allied independents control 46 seats in the Senate.
The hearing unfolded as Senate Majority Leader Mitch McConnell, R-Ky., demanded the administration immediately turn over the text of side agreements between Iran and the International Atomic Energy Agency. "The law is clear," he said in remarks on the Senate floor.
Administration officials say that in the past, such agreements have not been made available. They also say U.S. officials are available to provide information about them in classified meetings.
In his testimony, Kerry said that when the negotiations began, experts calculated that it would take Iran only two to three months to produce enough material for a bomb, the so-called breakout time. Tehran has always insisted that its nuclear program is civilian, not aimed towards nuclear weaponry.
"If the deal is rejected, we return immediately to this reality, except that the diplomatic support we have been steadily accumulating in recent years would disappear overnight," he said.
The United Nations Security Council has already voted to lift the international sanctions in place, effectively accepting the deal that the United States and other powers have struck with Iran. As a result, administration officials say the United States would be left trying to enforce more limited sanctions, without the support of other nations that backed the earlier steps.
"President Obama has made it crystal clear we will never accept a nuclear-armed Iran," Kerry said. "But the fact is that Iran now has extensive experience with nuclear fuel cycle technology. We can't bomb that knowledge away."
The Associated Press