As recently as last week, bipartisan congressional support for new sanctions on Iran, which were likely to sabotage diplomatic talks and move the world closer to a national security crisis, was growing, and momentum appeared to be on proponents’ side. As of this morning, the landscape looks far different

Its Senate sponsors describe it as a “diplomatic insurance policy” that will help President Obama cut a better nuclear deal with Iran. The White House condemns it as a deal-killer that could put the United States on a path to war. At issue is a 52-page Senate bill, the Nuclear Weapon Free Iran Act of 2013, which has become enshrouded in a fog of overheated talk, as the White House, Congress and a growing legion of lobbyists clash over the wisdom of passing new sanctions against Iran while pursuing diplomacy.

Iranian officials, who were brought to the negotiating table in the first place thanks to the efficacy of existing sanctions, have already said they’ll abandon diplomatic talks if the United States approves additional sanctions while negotiations are underway. In effect, Congress would be scuttling a historic opportunity, on purpose, without waiting to see whether diplomacy would work.

But the problems with the effort go even further. The proposed sanctions would also signal to the world that the United States is no longer interested in a peaceful solution and similarly disinterested in keeping our word. At the same time, the legislation represents a congressional effort to establish the parameters of any possible long-term deal with Iran – in effect, lawmakers would be imposing their own requirements on U.S. negotiators while talks are ongoing.

A week ago, it was practically a foregone conclusion that such a bill would pass the House and Senate; the question is whether President Obama’s veto could be overridden. Just in the last few days, however, the odds of such a bill even reaching the president’s desk have dropped unexpectedly.

The Hill, for example, reported yesterday that House Republicans “are moving away from a proposal to adopt new Iran sanctions.” House Democrats who were otherwise sympathetic to the idea became “irked” by GOP political tactics “and the idea appears to have been at least temporarily shelved.”

In the Senate, meanwhile, BuzzFeed reports that Sen. Bob Corker (R-Tenn.), a co-sponsor of the legislation, has “proposed the idea of scheduling a vote on Iran sanctions six months from now, after the interim nuclear agreement has run its course, instead of voting on sanctions right now.”

In other words, lawmakers could at least wait to see if the talks bear fruit before sabotaging them in advance. Corker’s idea isn’t ideal – it would reportedly lock in the Senate for a vote on July 21, exactly six months after the current deal is implemented, regardless of the status of the diplomacy – but in the larger context it suggests even sanctions supporters are starting to see value in waiting.

Indeed, an unnamed senator who supports the sanctions bill told Greg Sargent this week that opponents have the momentum. The senator added, “At the moment, there’s no rush to put the bill on the floor. I’m not aware of any deadline in anyone’s head.”

Keep in mind, the sanctions legislation was introduced in the Senate on Dec. 19 with a bipartisan group of 26 sponsors. Over the course of just three weeks, that total more than doubled to 59 sponsors. But the last addition was eight days ago – and no other senators have signed on since.

What changed the direction of the debate? To be sure, White House pressure has made a difference, reinforced by President Obama’s direct lobbying to Democratic senators this week. I also talked to a Senate staffer yesterday who said public pressure has also increased, with more voters contacting the Hill with phone calls and emails, voicing opposition to the bill.

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