A new NBC/Wall Street Journal poll, meanwhile, also shows support slipping. The poll, which offered a detailed description, now shows Americans are split evenly after they were clearly in support in prior months.
Just two weeks after it was struck, Americans appear to be taking an increasingly negative view of the Iran nuclear deal.
A CNN-ORC poll released Tuesday finds 52 percent of Americans say Congress should reject the deal, while 44 percent want them to approve it. The findings mirror a Pew Research poll released last week and mark the first public polls this year to find majority or plurality opposition to a deal with Iran.
Before the final deal was announced, polls found strong evidence for public support. A streak of eight public polls using very different question wording found support for a deal with Iran outpacing opposition. The margin of support for a deal (percentage support minus percentage opposition) ranged from four to 28 percentage points, but most showed the deal was popular by double-digit margins.
Polls after the announcement show a much more mixed bag. Besides the Pew and CNN polls finding a negative support-opposition margin of between eight and 12 percentage points, a Washington Post-ABC News poll released last week showed 56 percent supported the deal and 37 percent opposed it. Another post-deal poll from the Jewish Journal found a narrow split of 28 percent support to 24 percent opposition, with nearly half saying they don't know enough to say. (See below for results and wording for all polls.)
There is a lot of noise in comparing different polls on support for the Iran deal, as pollsters have asked different questions, making it hard to suss out changes in attitudes from changes in wording. Some ask approval/disapproval while others ask support/oppose. The latest CNN poll asks whether Congress should approve or reject the deal -- a different question than whether people actually support the deal themselves (although it wouldn't seem far different).
The Post-ABC and Jewish Journal poll are more specific about what the deal does (and find more support), while the Pew and CNN surveys provide less detail (and find greater opposition). The latest Post-ABC poll used similar language to describe the deal as in March, both finding largely positive margins of support (plus-19 and plus-28).
So what's going on here?
1. It's not just Republicans
The reasons for Americans more negative views of an Iran deal are not immediately clear, but the latest polls cast doubt on one theory we had: falling Republican support.
Attitudes on the Iran deal have been closely tied to partisanship, but pre-deal polls showed a surprisingly high level of support among Republicans, hitting 40 percent or more in three polls. The level of support persisted even after Republicans in Congress expressed near-universal opposition to the announcement of the deal's framework this spring.
We suggested Republican support might drop as party leaders spoke out against the final deal, dragging down overall support. But the latest polls show little increase in the partisan gap since the deal was announced; Democrats were on average 29 percentage points more supportive of the deal than Republicans before July 14, and 32 points more supportive afterward.
In fact, support for a deal is lower after the deal across party lines, among Democrats, Republicans and independents.
2. The devil's in the lack of details
A second possibility is that hearing the specifics of how a deal works make it more popular, and post-deal polls are seeking to measure opinions while providing minimal details -- a break from pre-deal surveys. The Pew survey only refers to a "recent agreement on Iran's nuclear program between Iran, the United States and other nations," and the CNN poll describes an agreement "aimed at preventing Iran from developing nuclear weapons."
By contrast, the Post-ABC poll finding greater support asked about a deal that would "lift economic sanctions against Iran in exchange for Iran agreeing not to produce nuclear weapons. International inspectors would monitor Iran’s facilities, and if Iran is caught breaking the agreement economic sanctions would be imposed again."
There's some evidence in pre-deal polling that less-detailed descriptions resulted in lower support. Both the AP-GfK and USA Today/Suffolk polls employed less-detailed language to describe the deal (see the Google doc below), and found support outpacing opposition by smaller margins than polls asking more detailed questions. The PPC poll provided the most detail about the background of a deal -- including several arguments for and against it -- and showed among the highest support for any survey for a deal as well as the most narrow partisan gap.
3. It got real
Lastly, the public might simply be reacting differently to a final deal -- warts and all -- than the idea of a deal in general. The public has long been skeptical about whether any deal will be effective, and those concerns have persisted even after the deal's announcement. The July Post-ABC poll found 64 percent saying they are "not so" or "not at all" confident the agreement will prevent Iran from developing nuclear weapons, and the Pew survey found public doubts of both the trustworthiness of Iran's leadership and the ability of the international community to monitor Iran's compliance.
Regardless of the reasons, public support for the final Iran deal is clearly weaker than before it was announced, making it a tougher sell for President Obama to solidify the one-third support in either chamber of Congress needed to keep it in-place.
See the Google doc below or at this link for detailed results and question wording on Iran polls.
Peyton M. Craighill contributed to this report.