Dan Drezner concludes that domestic political resistance to an Iranian nuclear deal will likely be too strong to overcome:

It’s also not obvious to me, by the way, that either President Obama or President Hassan Rouhani will be able to make the hard sell on a compromise to their respective legislatures. It’s not like Obama’s national security street-cred is riding terribly high at the moment, and Rouhani has his own hardliners to massage.

So the political scientist in me thinks that a nuclear deal would be good for the United States in the short and long runs. But that same political scientist in me is also increasingly skeptical about arguments that leadership will somehow be able to override hardliners in both countries to get to that deal.

The more immediate problem on the U.S. side is not whether such a deal can be “sold” to the Senate, since there will be no need for the Senate to vote on the deal itself (no matter what McCain et al. want), but whether there are domestic political changes in the meantime that make it more difficult to reach an agreement in the first place. The midterm elections will take place at the start of next month, and the current deadline to reach an agreement comes almost three weeks later. Depending on runoffs and victories by independent candidates, we may not know on November 5 which party will end up controlling the Senate. Even so, it is more likely than not that Republicans will have enough seats to control the chamber in the new year. That will shortly put them in a position to block sanctions relief, and it could lead to a renewed push for imposing additional sanctions. That wouldn’t matter so much if Senate Republicans were inclined to judge a nuclear deal on the merits, but most of them are ideologically opposed to making any deal with Iran on this issue. Even those that claim to want diplomacy to succeed insist on conditions, including zero enrichment, that are obvious deal-breakers for the Iranian side.

Since the Senate GOP is opposed to a final agreement with Iran that doesn’t include their impossible conditions, the prospect of their takeover could adversely affect the final stage of the negotiations this year. If the Iranians see that Republicans are poised to win control, they might be more likely to stall or walk away from the talks all together. In the worst case, there might be no deal because the Iranian side assumes that the U.S. won’t be willing to fulfill the rest of its side of the bargain, or there could be a deal reached that is then blown up a few months later as it becomes clear that there will be no action on sanctions relief from Congress.