“…If the administration signs an agreement with Iran, Congress or regional allies can do little to force it to back down. Nor should anyone try. The United States depends on a modicum of international law and smooth relations with key international players to run the global security system that protects us all. Once any administration enters into an international agreement — particularly with important states such as P5+1 partners Britain, China, France, Russia, and Germany — there are compelling arguments to stick with it absent breach of faith.

While Congress must be consulted on any agreement, the administration and the P5+1 states can lift UN sanctions against Iran on their own authority, while the president can invoke the 2012 National Defense Authorization Act (NDAA) to waive or suspend the most important U.S. sanctions — those penalizing third-country purchases of Iranian oil — on national security grounds.”

“Dealing with a Bad Iranian Nuclear Agreement”
http://www.washingtoninstitute.org/policy-analysis/view/dealing-with-a-bad-iranian-nuclear-agreement