Over the last several years, Congress has led the push for increasingly tough sanctions against Iran, and could approve even tougher measures that would drive Tehran away from any potential deal with the U.S. and other powers.
Israeli Prime Minister Benjamin Netanyahu also remains staunchly opposed. He argues that letting any centrifuges spin in Iran will allow scientists there to sharpen their mastery of nuclear science and edge toward bomb-making capability.
Israeli officials have talked of attacking Iranian's nuclear facilities before they are so advanced and hidden so deeply underground that they are invulnerable to bombs.
But Defense Minister Ehud Barak, in an interview last month with the Jerusalem Post, outlined goals that would allow Iran to retain some low-enriched uranium for nonmilitary purposes. He did not call for an end to all enrichment.
"There have been many signals lately that the red line has shifted and they're no longer pushing for full suspension," said Michael Singh, who served as President George W. Bush's top Iran advisor and who strongly opposes allowing Iran to enrich any uranium.
George Perkovich, a nonproliferation specialist at the Carnegie Endowment for International Peace, said he was among the U.S. hawks who believed until recently that "you have to hold the line on enrichment by Iran."
Now, he said, "that view has been overtaken by events." Iran has enriched more uranium, public support for the program is widespread, and the prospects of giving up all enrichment "has become a nationalist taboo in Iran."