A bid to slap Iran with a new round of economic sanctions appears to have stalled in the Senate, after leading Democrats amplified concern about the threat such a move poses to a fragile diplomatic process.

Early in the week, reports that a bill introduced by Republican Mark Kirk and Democrat Robert Menendez was within striking distance of a veto-proof majority cast a shadow over news that negotiators had finalized a temporary agreement to freeze Iran’s nuclear program, beginning Monday. New sanctions would likely kill negotiations for a final deal, the White House warned lawmakers, and increase the chances of an armed conflict with Iran.

But Senate majority leader Harry Reid has given no indication that he will bring the bill up for a vote, and the pressure to do so is falling now that top Democrats have intensified opposition to the proposed legislation. The Kirk-Menendez bill gained no new endorsements this week, and even one supportive senator admitted Wednesday to a break in momentum.

Dianne Feinstein, chair of the Intelligence Committee, called the sanctions bill "a march towards war" on Tuesday in a floor speech that was remarkable in detail and force. “I deeply believe that a vote for this legislation will cause negotiations to collapse,” Feinstein said, after thoroughly rebutting many of the claims about the interim deal put forth by the bill’s supporters. “The United States, not Iran, then becomes the party that risks fracturing the international coalition that has enabled our sanctions to succeed in the first place.”

Ten committee chairs circulated a joint statement warning that “new sanctions would play into the hands of those in Iran who are most eager to see the negotiations fail.” Majority Whip Dick Durbin, Jeff Merkley of Oregon, Chris Murphy of Connecticut, Bill Nelson of Florida, and Tim Kaine of Virginia said they opposed the measure at this point. Even one of the co-sponsors of the Kirk-Menendez bill, Richard Blumenthal, indicated that the time wasn’t right for a vote. “I want to talk to some of my colleagues. I’m encouraged and heartened by the apparent progress and certainly the last thing I want to do is impede that progress,” he said on Monday.

Major newspapers condemned the bill, including theNew York Times, the Los Angeles Times, USA Today, and the Washington Post, whose editorial board often betrays a neoconservative streak. More than sixty organizations including J Street, the National Iranian Council, American Baptist Churches, and CREDO delivered a letter to the Senate on Tuesday stating that the law “sets insurmountable demands for a comprehensive nuclear deal” and would “critically endanger the possibility of a diplomatic resolution to the nuclear standoff with Iran, increasing the likelihood of a nuclear-armed Iran and an unnecessary and costly war.”  

The Kirk-Menendez bill has 58 co-sponsors, and the very real chance that it would pass the Senate if given a vote remains concerning. But it’s important to note that rumors that the legislation has enough support to override a potential veto comes from its backers, and so warrant some skepticism. Although 16 Democratics co-sponsored the legislation, the sanctions push has grown increasingly partisan, with Republicans nearly unanimous in their support. Of the 25 senators who have signed on to the bill this year, only one is a Democrat.

The gorilla in the room is the American Israeli Public Affairs Committee, which has been calling for new sanctions for months. Of the 16 Democrats who have endorsed the Kirk-Menendez legislation, several are up for re-election in closely contested states; Senator Kirk himself suggested Tuesday that a vote for new sanctions would be an opportunity for lawmakers to shore up support from the powerful lobby. “The great thing, since we represent a nationwide community — the pro-Israel community is going to be heavily present in most states — this is a chance for senators to go back and tell them, ‘I’m with you,’” Kirk said. Other Democrats pushing for the bill have close ties with the group, particularly Chuck Schumer and Cory Booker.

Tellingly, the Kirk-Menendez bill states that if Israel takes "military action in legitimate self-defense against Iran's nuclear weapons program,” the US "should stand with Israel and provide…diplomatic, military, and economic support to the Government of Israel in its defense of its territory, people, and existence." The language is nonbinding, but it raises flags about whose interests the legislation would truly serve.

Dianne Feinstein addressed this point more directly than perhaps any other politician so far. “While I recognize and share Israel’s concern, we cannot let Israel determine when and where the US goes to war,” she said. “By stating that the US should provide military support to Israel should it attack Iran, I fear that is exactly what this bill will do.”

Such outspokenness about the relationship between US policymaking in the Middle East and Israeli interests is remarkable. But other lawmakers are signalling that they too are shrugging off the lobby: Democratic Representative Debbie Wasserman-Shultz, normally a high-profile ally for AIPAC, reportedly argued against the Kirk-Menendez bill at a White House meeting attended by several dozen of her colleagues on Wednesday night. How things play out in the next week, and in the duration of the talks with Iran, will be a good test of AIPAC’s influence, which seemed diminished when Congress considered military strikes in Syria last year. Progressives claimed a victory when diplomacy prevailed then; as Peter Beinart points out, the current debate presents a real opportunity for the anti-war left to reassert itself, not only to punish lawmakers who start wars, but to set new expectations for a diplomacy-first approach.

Feinstein asked a good question in the opening of her speech: “Can, in fact, a country like Iran change?” Congress is now weighing whether to double down on a fourth decade of economic screw-tightening when it comes to Iran, a tack that looks increasingly untenable. What’s really being asked is whether things can change at home.