A survey of voters by Monmouth University released this month pointed to the unravelling, with a mere 27% believing that US Congress should approve the deal, despite the strong advocacy of President Barack Obama.
As could be expected, 55% of Republican voters reject the deal, as do 33% of independents; but it's the low confidence of Obama's own Democrats that is most concerning analysts, with only 41% of them believing that Congress should approve the deal.
West Australian independent research institute Future Directions International said it was "conceivable that the sentiment that cuts across party lines stems from a wider distrust of Iran and its motives".
In response to the question "How much do you trust Iran to abide by the terms of this agreement - a lot, a little, or not at all?" - 61% said they did not trust Iran at all; while just 6% said they did trust Iran "a lot" - a figure which has not grown hugely from 4% last year.
This poll came out a week after an NBC poll, conducted soon after the plan was announced, which revealed an almost even split: 35% supported the deal, 33% opposed it and 32% weren't sure as they didn't know enough about the terms of the agreement.
More concerning, however, is the Monmouth survey revealing that only 14% of Republicans and Democrats believe the US came out on top in the deal.
FDI said the latest poll laid to rest Obama's claim that he would've won the next election had he been allowed to stand for a third term, and "served to further undermine his credibility".
The deal also faces significant opposition on the other side, with Iran's Majilis (parliament) passing a bill called Government's obligations to preserve Iran's nuclear achievements on June 23 which basically says any restrictions placed on Iran by the international community must comply with its own policies.
Undeterred, France and Iran have rapidly increased their affection for each other, eyeing the estimated $US150 billion ($A205.04 billion) of frozen Iranian funds that will be freed up as part of the terms of the deal.
"Paris will, no doubt, wish to sell its nuclear technology to Iran in addition to its military hardware," FDI said, while Russia would also like to sell more nuclear reactors and military hardware to Iran.
The other key player in all this is China, which is likely to concentrate for the present on purchasing Iranian energy products for its own strategic reasons, notably to diversify its energy sources and increase its presence in the region, FDI said.
China has already entered into an agreement with Tehran to pipe Iranian oil and gas via Pakistan into its western regions; while Beijing has, in turn, offered to build nuclear reactors for Tehran using its own technology.
"Much, then, rests upon the decision Congress makes in regard to the deal," FDI concluded.
"In the final analysis, however, it is likely that international events will not have as much bearing upon their decision as domestic sentiment: the Republicans desperately want to win the next general election.
"This one fact, by itself, is likely to see the Iranian Plan of Action scuttled."
