With Iran threatening that any new sanctions would scuttle its interim nuclear deal with the West, the Obama administration is fighting a fierce battle to convince skeptical Senate Democrats not to pass any new measures against Tehran.
The White House effort achieved some success on Tuesday, when Senator Tim Johnson, a South Dakota Democrat who is chairman of the Senate Banking, Housing and Urban Affairs Committee, said the administration had made a strong case for a "pause" in congressional action, and that he was inclined not to move sanctions legislation forward in his committee.
But other prominent Democrats, including Senator Charles E. Schumer of New York, support new sanctions, with deferred imposition, and Senator Harry Reid of Nevada, the majority leader, is said to be sympathetic.
Proponents of sanctions said Mr. Zarif's threat was hollow since, by their estimates, Iran's economy would suffer heavy damage if the negotiations collapse. Mark Dubowitz, executive director of the Foundation for Defense of Democracies, said Iran would lose $9 billion in oil revenue, on top of what it has already lost, in six months.
"The reality is that a no-deal option is to watch his economy crater," Mr. Dubowitz said. "If he does walk away from a sanctions-in-waiting bill, then Khamenei was never serious," he added, referring to Iran's supreme leader, Ayatollah Ali Khamenei.