WASHINGTON — 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.
Iranian officials have threatened to leave the bargaining table if the United States enacts any new sanctions during the negotiations.
The White House has cast the issue in stark terms, saying that a vote for new sanctions would be, in effect, a “march toward war” and challenging those lawmakers who support the bill to acknowledge publicly that they favor military action against Iran.
“It just stands to reason if you close the diplomatic option, you’re left with a difficult choice of waiting to see if sanctions cause Iran to capitulate, which we don’t think will happen, or considering military action,” said Benjamin J. Rhodes, a deputy national security adviser.
Yet senators from both parties angrily reject that characterization, saying that congressional pressure to impose sanctions is what brought Iran to the negotiating table to begin with. If anything, they said, the West needs the specter of more sanctions as a “diplomatic insurance policy,” in case Iran reneges on the interim deal or the talks ultimately fail.
Behind these positions is a potent mix of political calculations in a midterm election year. Pro-Israel groups like the American Israel Public Affairs Committee, or Aipac, have lobbied Congress to ratchet up the pressure on Iran, and many lawmakers are convinced that Tehran is bluffing in its threat to walk away from the talks.
The signing of the interim agreement, congressional aides said, could cut both ways. While some senators might be more inclined to give diplomacy a chance, others might be troubled by the terms of the six-month deal. The full text of the agreement has not yet been released, arousing the suspicions of critics, though the White House said on Monday that it would soon be made available to lawmakers.
Mr. Obama and other senior officials have met repeatedly with lawmakers to defend their diplomatic efforts and to try to stop the rush to sanctions. They cite an intelligence assessment that sanctions could undermine the negotiations. And they argue that Congress can always act swiftly to impose sanctions if the talks do collapse.
“My preference is for peace and diplomacy, and this is one of the reasons why I’ve sent the message to Congress that now is not the time for us to impose new sanctions,” Mr. Obama said to reporters on Monday after meeting with Spain’s prime minister, Mariano Rajoy. “Now is the time for us to allow the diplomats and technical experts to do their work.”
Much will depend on the Senate majority leader, Harry Reid of Nevada, who has so far resisted pressure to allow a vote on a sanctions bill. Mr. Reid is balancing a record of robust support for Israel with an equally strong alliance with the White House. Aides say Mr. Reid will not bring a bill to the floor before the State of the Union address on Jan. 28.
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Democrats, they say, recognize the delicacy of Mr. Obama’s signing a veto on the Iran bill, especially if Congress delivers the first veto override of his presidency on a matter that is so clearly a presidential prerogative. But Democrats said the current lull can hold only for a matter of weeks, not months.
Prominent Democrats like Senator Michael Bennet of Colorado, the chairman of the Democratic Senatorial Campaign Committee, have recently thrown their support behind the bill. Aides say enough Senate Democrats would support the sanctions bill to override a presidential veto, and the House probably has a veto-proof margin as well.
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.”
Reports of an oil-for-goods swap being negotiated between Iran and Russia have prompted some Democrats to accuse Tehran of violating the terms of the interim deal. The White House said it shared those concerns, noting that Secretary of State John Kerry raised the matter with Russia’s foreign minister, Sergey V. Lavrov, on Monday.
Such a deal would be “inconsistent” with the agreement and “could potentially trigger U.S. sanctions,” said a spokeswoman for the National Security Council, Caitlin M. Hayden.
There is also a spirited debate about whether the Senate legislation can be reconciled with a nuclear deal. A study by the Center for Arms Control and Non-Proliferation said the bill “moves the goal posts” by making a deal with Iran contingent on Iran’s not launching ballistic missiles, and requiring it to have no involvement in terrorism against the United States, directly or through proxies.
Senator Carl Levin of Michigan, chairman of the Armed Services Committee and a vocal opponent of the bill, said the signing of the accord strengthened his position. “It should make it harder for people to act in a way which might undermine the chances of there being a comprehensive agreement,” he said.
As the debate has intensified, some Democrats are taking umbrage at the White House’s tone, pointing to a statement last week from a National Security Council official that said, “If certain members of Congress want the United States to take military action, they should be up front with the American public and say so.”
Senator Benjamin L. Cardin, Democrat of Maryland and a strong supporter of the legislation, bristled at the White House’s pressure, especially its “march to war” language. “I think they should regret using that language,” he said. “The bad actor is Iran.”
An article on Tuesday about President Obama’s struggle to prevent Senate Democrats from backing a bill that could thwart his diplomatic efforts with Iran referred incorrectly to Mr. Obama’s threatened veto of the legislation. If he vetoes the bill, it will be the third veto of his presidency, not the first. (He vetoed the Interstate Recognition of Notarizations Act of 2010 and a Continuing Appropriations bill in 2009.)