In an editorial published by Politico, Samantha Power argued that a "no" vote from Congress would make it more difficult for the United States to drum up support for sanctions and partner with like-minded countries to confront crises.
"If the United States rejects this deal, we would instantly isolate ourselves from countries that spent nearly two years working with American negotiators to hammer out its toughest provisions," Power wrote in a piece posted late Wednesday.
The US Congress is due to vote next month on whether to endorse the deal reached in July between Iran and six world powers -- Britain, China, France, Germany, Russia and the United States.
The agreement would lift crippling UN sanctions against Tehran in exchange for curbs on Iran's nuclear program.
Power argued that rejecting the deal would undermine the US's ability to seek sanctions in other situations because it would convey an image of the United States as "a superpower intent on inflicting pain for its own sake."
"The price of our lonely walk away looks very high indeed," she said.
