With the United States and Iran about to embark on a critical phase of nuclear talks, President Obama is waging an intense rear-guard action to prevent Senate Democrats from supporting strict new sanctions that could upend his diplomatic efforts.
Sponsors of the bill, which would aim to drive Iran's oil exports down to zero, have secured the backing of 59 senators, putting them within striking distance of a two-thirds majority that could override Mr. Obama's threatened veto. Republicans overwhelmingly support the bill. So far 16 Democrats have broken with the president, and the bill's sponsors hope to get more.
The struggle is casting a long shadow over the talks, which administration officials say will be even harder than those that resulted in the six-month interim agreement, signed Sunday, that will temporarily freeze Iran's nuclear program in return for limited sanctions relief.
The fate of the bill, some on Capitol Hill said, is likely to rest with news from Iran. If newspapers begin running front-page articles about a resurgent Iranian economy or news breaks of burgeoning trade between Iran and its allies, Mr. Reid may be pressured to allow a vote. Proponents of sanctions say there is already ample evidence of both.
"The Iranian economy has stabilized and is now starting to go on a positive trajectory," said Mark Dubowitz, the executive director of the Foundation for Defense of Democracies. "This is due in a significant way to changing market sentiment."