Sceptical politicians are demanding a raft of tougher US penalties against Iran if a landmark nuclear deal falls apart.
Tehran has agreed to a six-month pause in its nuclear programme while diplomats continue talks to prevent Iran from developing a nuclear weapon. International observers will monitor Iran's nuclear sites and ease about seven billion dollars (£4.3bn) of the crippling economic sanctions.
But the announcement, after months of secret talks between the United States and Iran, left many US politicians deeply doubtful of the most significant agreement between Washington and Tehran in more than 30 years of estrangement.
Chairman of the Senate Foreign Relations Committee, Democrat Bob Menendez, said he would work with colleagues to have sanctions against Iran ready "should the talks falter or Iran fail to implement or breach the interim agreement".
Such distrust that Iran was negotiating in good faith ran across the political spectrum in a congress that is otherwise deeply divided. And ready-to-go sanctions seemed to have rare cross-party support across both congress chambers.
President Barack Obama convinced the senate leadership to hold off considering the measure while negotiators pursued an agreement. Senate Democratic leader Harry Reid agreed to the request but said his chamber would take up new sanctions in December - with or without an agreement with Iran.
Senator Chuck Schumer, a member of his party's leadership team, said he was "disappointed" by the deal, which he called disproportional. The New York Democrat said sanctions forced Iran to negotiate and he planned further discussions with colleagues.
"This agreement makes it more likely that Democrats and Republicans will join together and pass additional sanctions when we return in December," he said.
Deputy White House press secretary Josh Earnest said new sanctions would undermine the international coalition the United States had built around the talks.
The Senate returns on December 9 and members are already talking about sanctions designed to warn Iran that failure to use the six-month window to reach a deal would only leave its people in worse economic straits.
"Congress, I think, will want to make it clear that if Iran does not live up to these commitments, we will not only insist that the sanctions be reapplied, but we will have stronger sanctions against Iran," said Democratic senator Ben Cardin.
Republican senator Marco Rubio said: "There is now an even more urgent need for Congress to increase sanctions until Iran completely abandons its enrichment and reprocessing capabilities."
The House of Representatives' number two Democrat, Steny Hoyer, said the threat of even tougher sanctions could help keep Iranian diplomats at the negotiating table for talks designed to prevent Tehran from being able to produce a nuclear weapon. He said he supported having the sanctions ready to go in case Iran proves an unreliable negotiating partner.
"It is appropriate that we wait six months to implement those, which will say to the Iranians, 'We need a final deal, and if not a final deal, these tougher sanctions are going to go into place'," he said.
Washington first sought to apply economic pressure to Iran after protesters seized the US embassy in Tehran in 1979 during the Islamic Revolution. An escalating series of sanctions followed, eventually crippling Iran's economy and putting pressure on the nation's middle class. Many of those economic penalties are set to remain in place during the six-month negotiating window announced yesterday.
But politicians seemed to expect talks to collapse, forcing them to restore old sanctions and levy new ones.
"If Iran does not consent to a comprehensive agreement that ensures it cannot acquire a nuclear weapon, there is a broad consensus in congress to impose even tougher sanctions," said Democrat Carl Levin, chairman of the Senate Armed Services Committee.
A deep distrust of Iran pervaded yesterday's discussion of the deal. "We need to be very, very careful with the Iranians," said Eliot Engel, the top Democrat on the House Foreign Affairs Committee.
"I don't trust them, I don't think we should trust them. ... Sanctions should always be hanging there because that's what brought Iran to the table in the first place."
Republican Ed Royce, who chairs the committee, said Iranians "are capable of cheating" and Republican House of Representatives speaker John Boehner said the six-month pause deserved healthy scepticism.
"Iran has a history of obfuscation that demands verification of its activities and places the burden on the regime to prove it is upholding its obligations in good faith while a final deal is pursued," he said.
Republican Mike Rogers, who chairs the House of Representatives intelligence panel, was more forthright about the deal, which he said aids "the leading nation state of terror".
"We have just rewarded very bad and dangerous behaviour," he said.
Some politicians have compared the current deal with the 1990s pact that gave North Korea a respite from sanctions if it promised to stop work on its nuclear programme. "We've seen what's happened in North Korea; they now have nuclear weapons. And I don't want to see that happen in Iran," said Republican senator Bob Corker.