American public support has fallen, but the agreement remains uncontroversial across Europe

President Barack Obama is urging supporters to back the Iran nuclear deal as opponents begin a well-funded campaign to block the deal. The agreement, which aims to prevent Iran from developing a nuclear weapon in return for sanctions relief, is likely to receive a vote of disapproval from the majority of the US Congress. Obama can veto such a vote, but he will need the support of at least a third of members in either Chamber to sustain a veto.

New polling by YouGov America finds support there waning. Just 36% of the public say they back the deal in the latest poll, down from 51% in YouGov's previous survey. 38% now oppose the deal, up from 33% in the previous poll, while the number who are unsure how they feel about the deal has increased from 16% to 26%.

Political polarisation explains some of the change. Amid escalating rhetoric and unified opposition among Republican Party leaders, Americans who identify as Republicans now overwhelmingly oppose the deal (71% to 17%) while supporters of Obama’s Democratic Party support it by 62% to 11%. Independents, once supportive, are now much more likely to be undecided.

Europe

The agreement was finalised by representatives from six major powers and Iran this July.

One of President Obama’s arguments in favor of the deal has been that it is backed by the world community. A new survey of seven European nations, including three countries whose governments helped negotiate the deal itself (the United Kingdom, France and Germany) shows that Europeans do tend to be far more favourable to the deal than Americans.

Support for the Iran nuclear deal is highest in Germany, where 68% approve of it, but even where support is weakest (Finland and Sweden) nearly six in ten are behind it.

The latest finding show little change from previous surveys in Britain and Germany.

Europeans back the agreement despite doubts about its chances of success, also shared by many Americans. Between 50% and 63% in each European country lack confidence the deal will prevent Iran from developing a nuclear weapon.

Yet while 72% of Americans see Iran's nuclear program as a serious threat, the number of Europeans who feel threatened is much lower, from 53% in France to just 15% in Finland.

Full US poll results can be found here and topline results and margin of error here.

Full European results can be found here.