Some version of this lies behind Menendez’s argument – if this theory is right, then political divisions back home can make a negotiator stronger, not weaker. They allow the negotiator to argue that her hands are tied, so that she can avoid concessions that she would otherwise have to make. Menendez is arguing that the Iranians are using their own domestic divisions to strengthen their hands in negotiations. Because any agreement will have to satisfy Iranian hardliners, the Iranian negotiators are able to play tough, and extract benefits from the U.S.
Menendez seems to think that the US needs its own hardliners too, to narrow the U.S. win set, so that it doesn’t make concessions that it shouldn’t be making. New sanctions legislation would make it clear to Iran both that Senate ‘ratification’ (i.e. passing legislation to roll back sanctions) is going to be very hard and that failure to reach an acceptable deal will hurt Iran.
The problems with this line of reasoning are twofold. First, there isn’t any very good evidence that it works on its own terms. In the words of Peter Evans:
The strategy of ‘tying hands’ – deliberately shrinking the win-set in pursuit of an agreement closer to the [negotiator’s] preferred outcome – is infrequently attempted and usually not effective. The ‘tying hands’ strategy, suggested by Thomas Schelling’s work, is logically plausible but lacks efficacy in practice. Perhaps because they are aware of its limited efficacy, statesmen prefer not to have their ‘hands tied’ by constituents, even when they share the constituents’ preferences
Second, it can be very risky. It can easily go too far, by shrinking the perceived win set down so much that there is no longer any possible agreement. Any final deal will have to get sign-off from the House and Senate, which will both have to roll back the sanctions legislation that is already in place. New sanctions may lead the Iranians to conclude that there isn’t any possible deal that would pass the House and Senate and also be acceptable to Iran.
Plausibly, shrinking the win set too much will be riskier when the actors doing the shrinking don’t have a detailed grasp of the nuances of the negotiations. The risk will obviously be even higher if these actors have to compromise with others who sincerely want the negotiations to fail. Both conditions seem to apply to the Senate’s threats.
It’s certainly possible that Menendez is right – that hardline posturing by the Senate will influence negotiations to the benefit of the U.S. without any risk. However, it’s probably not the way you should be betting.