There's one incontrovertible point in the argument against the Congress rejecting and invalidating the nuclear agreement with Iran. Even if you think the deal was poorly negotiated, if the U.S. walks away the sanctions regime will immediately fall apart and Iran will gain everything it could ever gain either by cheating on the deal or stepping up its enrichment and weapons program when various constraints expire.
Obama has made this point repeatedly, minus the "even if you think..." part. The shorthand is "what is your alternative?" -- and there is none. Implicitly recognizing this, Chuck Schumer, in the most self-negating policy statement I've ever read, could barely bring himself to sketch in (in the statement's last breath) an alleged alternative path: