Addressing the possibility of Congress blocking the Iran nuclear deal with the P5+1, White House press secretary Josh Earnest warned that such a move would isolate the United States internationally, and would render US sanctions against Iran, many of which are supposed to be lifted by the deal, effectively unenforceable.
Earnest went on to say that the sanctions were what brought Iran to the table to negotiate in the first place, and if the US Congress kills the deal, they would get “all the benefits of this deal without having to give up anything.”
The argument makes some sense, as the US sanctions were grudgingly supported internationally on the idea that it would lead to a deal like the one finally reached. If the US reneged on the internationally-backed deal after finally getting it done, many nations would probably balk at letting the US sanctions return unchallenged.
At any rate, this is seen as extremely unlikely, as there would need to be a super-majority in both houses of Congress to block the deal, and despite some claims to the contrary from hawks and a lot of money being poured into the campaign by the Israel Lobby, that’s likely an unattainable number of Congressmen to sway.