The United States risks a collapse in its future negotiating credibility if it walks away from the Iranian nuclear deal, according to US secretary of state John Kerry, who argues Congress now has little choice but to back the plan.



In his starkest warning yet to critics on Capitol Hill, Kerry also predicted the international coalition behind economic sanctions would quickly collapse and leave Iran even closer to developing an atomic bomb if the US sought to renegotiate terms.



One hundred senators and 535 members of the House of Representatives will vote in September on whether to oppose the deal struck by the coalition in Vienna, which proposes lifting sanctions in exchange for increased inspections and various restrictions on Iran’s nuclear activities.



But with sceptical Republicans struggling to persuade enough Democrats to join them in overriding a presidential veto, Kerry painted a bleak picture of the alternative options during four hours of heated testimony before the House foreign affairs committee on Tuesday.



“If Congress rejects this, Iran goes back to its enrichment. The Ayatollah will not come back to the table ... the sanctions regime completely falls apart,” he said.



“We will have set ourselves back. I don’t know how I go out to another country if that happens and say: ‘Hey, you ought to negotiate with us,’ because they will say: ‘Well, you have 535 secretaries of state in the United States. We don’t know who we are negotiating with. Whatever deal we make always risks being overturned.’”

Asked why the White House did not seek to ratify the deal as a treaty through the Senate alone, Kerry was scathing about its oversight function.

“I spent quite a few years trying to get treaties through the US Senate, and frankly, it’s become physically impossible,” he said. “You can’t pass a treaty anymore.”

The uncompromising message appeared to go down badly with many members of the committee, including Democrats who questioned whether Iran could be trusted to uphold the deal and whether tougher inspection requirements could have been agreed.

“You can’t do a good deal with a bad guy,” said Michigan Republican David Trott.

New York Republican Lee Zeldin said: “There is another alternative other than war; it’s a better deal ... America got played like a five string quartet.”

Like a Senate version last week, the lengthy hearing got increasingly tetchy, with both sides frequently interrupting each other, and Kerry – who entered on crutches after breaking his leg cycling – was forced to stand and stretch at times.



But the secretary of state was adamant that the deal was the best available and was no longer up for negotiation.

“If we walk away, we walk away alone,” he said. “Our partners are not going to be with us. Instead, they will walk away from the tough multilateral sanctions that brought Iran to the table to begin with. Instead, we will have squandered the best chance we have to solve the problem through peaceful means.”

The former Massachusetts senator also dismissed the idea that military strikes were a realistic way of containing Iran’s nuclear potential.

“Iran has already mastered the fuel cycle,” he said. “They have mastered the ability to produce significant amounts of fissile material. You can’t bomb away that knowledge any more than you can sanction it away.”

The tone of the administration’s pitch to Congress appears to have shifted in recent weeks from actively selling the merits of the deal to stressing the lack of viable alternatives, but it continues to argue that fears among Republicans and the Israeli government about the impact of lifting sanctions are overstated.

“Iran is in a massive economic hole from which it will take years to climb out,” treasury secretary Jack Lew told the committee.