The letter drafted by the freshman Senator Tom Cotton, and signed by 46 of his Republican colleagues, warning Iranian Supreme Leader Ayatollah Ali Khamenei that any deal he makes with U.S. Non-Supreme Leader Barack Obama might soon be undone by right-minded American legislators, is problematic on a number of fronts. But let me address just a couple, in the interest of time (I'm off to Germany to give a couple of talks on—of all things— the manner in which American foreign policy is made), and because others (including Dan Drezner) have addressed some of the broader issues already.
Here are two things I think are true about the current Iran nuclear negotiations:
1) These talks give the West the best, and least bloody, opportunity possible to keep the Iranian regime far from the nuclear threshold;
2) There is still a decent chance that Ayatollah Khamenei will ultimately balk at the conditions set by the U.S. and its allies, and wind up subverting the talks. Remember, as weak as this still-theoretical deal seems to some people (I'm in the camp of the moderately worried on this front), the conditions laid down by the West may be seen by the viciously anti-American ayatollah as too humiliating to accept. For instance, the deal might break apart over the issue of sanctions relief. The Iranians want to see most sanctions removed immediately upon completion of the deal; the West, of course, wants to phase out sanctions gradually, in order to ensure Iranian compliance with the terms of the deal.
The deal could also come apart for any number of other reasons. At which point—here's the important part—it's vitally important for President Obama to be able to argue to America's sanctions partners in Europe and Asia that the U.S. did everything possible to strike a deal, and to keep Iran at the table. If the talks collapse, President Obama will have to maintain, and strengthen, the broad-based sanctions coalition he built earlier in his term, with help from Congress and his erstwhile sidekick, bad-cop Bibi Netanyahu. But if America's partners—particularly those in Asia—come to believe that it was the U.S., rather than the Iranian leadership, that subverted the talks, the cause of invigorated sanctions will be damaged, possibly grievously.
It is not in the best interest of the United States to provide Iran any excuses to walk away from the table, and to provide Russia, China, and America's various European and Asian allies with arguments against strengthened sanctions. The smug, stomach-churning statement from the Iranian foreign minister, Mohammad Javad Zarif, in response to the Republican letter can be understood as a preemptive attempt to blame a future negotiations collapse on the U.S.
Republicans in the Senate may believe they were doing the U.S. a favor by issuing their warning to Ayatollah Khamenei, but advocates of crushing sanctions against Iran might just have undermined their own cause.