US President Barack Obama has predicted that opposition to his nuclear deal with Iran will erode as the agreement is implemented and a "parade of horribles" fails to materialize.

In an interview recorded before Obama left on vacation Friday, he said Ronald Reagan faced similar Republican criticism when he decided to talk to the Soviet Union's Mikhail Gorbachev.

"His conservative supporters wrote some really rough stuff about him as appeasing the evil empire," Obama said, framing a very immediate Congressional fight in broader historic context.

Obama is battling to secure enough votes in Congress to make sure the deal survives and avoid a humiliating defeat.

"When this agreement is implemented and we've seen centrifuges coming out of facilities like Fordow and Natanz, and we've got inspectors on the ground and it becomes clear that Iran in fact is abiding by this agreement, then attitudes will change," Obama told National Public Radio.

"People will recognize that, in fact, whatever parade of horribles was presented in opposition have not come true," he added.

Critics argue that the deal -- exchanging sanctions relief for Iran's scaling back its nuclear program -- will offer Tehran a cash windfall, while leaving key nuclear assets in place.

A vote in Congress is expected in September.