Washington - Secretary of State John Kerry will present his best case to Congress against ploughing ahead with new economic sanctions on Iran that could break a historic nuclear agreement and estrange America's closest allies in Europe as well as China and Russia.
His scheduled testimony on Tuesday before the House Foreign Affairs Committee comes as lawmakers threaten to undermine last month's interim nuclear accord in Geneva. There, the US and world powers promised Iran $7bn in sanctions relief in exchange for nuclear concessions.
The deal prohibits the Obama administration from introducing new sanctions for six months. Kerry and other US officials have warned of dire consequences if Washington breaks its word. And Iran's foreign minister, Mohammad Javad Zarif, has said any new package of commercial restrictions would kill the deal.
"If Congress adopts sanctions, it shows lack of seriousness and lack of a desire to achieve a resolution on the part of the United States," Zarif told Time magazine. "My parliament can also adopt various legislation that can go into effect if negotiations fail. But if we start doing that, I don't think that we will be getting anywhere."
Kerry also is concerned new legislative action would be a sign of bad faith to America's negotiating partners, which were Britain, France, Germany, China, Russia. The US is banking on them to enforce existing oil and financial restrictions on Tehran and to press Iran into a final nuclear deal next year.
Appearing at a weekend think-tank forum, President Barack Obama said he believed the chances for such a comprehensive nuclear agreement are 50-50 or worse.
Still, he defended diplomacy as the best way to prevent Tehran from acquiring atomic weapons.
He also rejected criticism from Israel's government and many in Congress that his administration bargained away too much without securing a complete halt to Iran's nuclear programme - as demanded by the international community for several years.
Obama said the interim agreement forces the Islamic republic to eliminate higher-enriched uranium stockpiles, stop adding new centrifuges and cease work at a heavy water reactor that potentially could produce plutonium. It also provides time to see if the crisis can be averted through negotiation, he said.
Iran insists its programme is solely for peaceful nuclear energy and medical research.
In Washington, Republican and Democratic lawmakers are leery.
They credit crippling petroleum, banking and trade sanctions levied on Iran in recent years with bringing its more moderate president, Hassan Rouhani, to power and his representatives to the negotiating table. They argue more pressure, not less, could break Iran's will and secure better terms in a final agreement.