Testifying publicly before Congress for the first time since world powers reached the landmark accord with Iran last week, America's top diplomat was confronted head-on by Republican accusations that Iranian negotiators had "fleeced" and "bamboozled" him.
The vitriolic exchanges at the Senate Foreign Relations Committee, which Kerry once chaired, reflected a hardening of positions as Congress opened a 60-day review of the deal considered crucial to its fate.
Iranian hardliners are also trying to undermine the pact, which U.S. ally Israel calls a dire security threat.
Kerry insisted critics of the agreement, which curbs Iran’s nuclear program in return for sanctions relief, are pushing an alternative he dismissed as a "sort of unicorn arrangement involving Iran's complete capitulation."
"The fact is that Iran now has extensive experience with nuclear fuel cycle technology," the former senator said. "We can't bomb that knowledge away. Nor can we sanction that knowledge away."
On crutches from a cycling accident, Kerry entered the hearing room to cheers from the anti-war group Code Pink.
Kerry said that if Congress rejects the accord, "the result will be the United States of America walking away from every one of the restrictions we have achieved, and a great big green light for Iran to double the pace of its uranium enrichment."
"We will have squandered the best chance we have to solve this problem through peaceful means," he said.
The 4-1/2-hour-long hearing was part of an intense Obama administration push to convince Democrats in particular to back the deal. Treasury Secretary Jack Lew and Energy Secretary Ernest Moniz also testified.
The three cabinet secretaries briefed the full House of Representatives and Senate behind closed doors on Wednesday and met privately with House Democrats after Thursday's hearing.