"I have a general policy on big issues like this: not to anticipate failure. And I am not going to anticipate failure now because, I think, we have the better argument," he said.
"I just go back again and again to those who are opposed to the deal, can't just say we want a better deal. They can't just say we're going to be tougher. This is serious," Obama told CNN's Fareed Zakaria.
Britain, France, Germany, Russia, China and the US signed an agreement last month with Iran to ensure Tehran does not acquire a nuclear bomb, in return for relief from sanctions that were wearing down its economy.
Obama said if Congress were to reject the Iranian nuclear deal deal, then that central goal would be harder to achieve.
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"And the international unity that we've brought about over the last several years would fray, not just with respect to sanctions, but with respect to the world's attitude about US leadership and how they gauge who's at fault in this dispute between the US and Iran," he said.
"That's continually tested. And what Congress needs to understand is, is that we are the most powerful country on Earth. But our power does not simply come from the fact that we've got the biggest military.
"Our power derives from the fact that since World War II, we have put together international institutions that have served our interests but have also served the interests of the world," Obama said.
He said: "if we stop doing that, then our power will be diminished, no matter how big our military budget is. And it will become a much more dangerous world. That's why I don't intend to lose on this.