Here is how rejection would play out.
First, our allies would desert us. This is not just an agreement struck between the United States and Iran. It is a deal negotiated over two years by the world powers. America led the way, but Russia, China, the conservative governments of Britain, France and Germany, and the entire European Union were equal partners. Everyone had to agree on every term or there would have been no deal.
Opponents spin fanciful notions of a "better deal" with tougher terms, bigger sticks. This is nonsense. Our European partners have already told us that it is this option or nothing. If Congress blocks the deal, no nation, least of all Iran, will believe that the United States is capable of making and keeping a new agreement. U.S. credibility would collapse faster than the Chinese stock market.
The sanctions regime would then unravel. The U.S. persuaded most of the world to curtail their trade and financing with Iran because we presented a feasible path to a diplomatic solution. Take away diplomacy and the sanctions cannot hold. Any new ones passed by Congress would be feckless.
With diplomacy over, sanctions withering and the hard-liners in ascendancy, Iran's nuclear program would come back with a vengeance.