There really is no precedent for what Cotton and his colleagues have done this week:

“Neither the Senate nor the House has sought to interfere with actual conduct of negotiations by writing an open letter to the leadership of a country with which the U.S. is negotiating,” said [Alan] Henrikson, who teaches at Tufts’ Fletcher School of international affairs.

Scholars can’t find precedents for this behavior because it had been generally understood and accepted up until now that this was something that members of Congress should not do. Opposition parties may rally to kill treaties that presidents have negotiated, but they don’t inform the other government in advance that the U.S. will quickly break its promises while the talks are still happening. Doing this puts no additional pressure on the other side, and cuts our own diplomats off at the knees, so it is an idiotic thing to do unless one desires to wreck negotiations. Cotton has made clear that this is exactly what he wants, and by signing off on the letter most of the Senate Republicans have joined with him in this. This is a very peculiar sort of hard-liner maneuver in that it emphasizes that our side is the one that shouldn’t be trusted to honor its agreements. Hard-liners normally insist that it is the other side will cheat on any agreement, but in a new twist Cotton and his Senate colleagues are boasting to the world that the U.S. can’t be trusted to keep its commitments in formal multi-party talks.

As Noah Millman says, Cotton and his allies are “deliberately trying to cripple America’s ability to conduct foreign policy.” That’s basically correct. The only qualification I would make here is that they are specifically trying to cripple the U.S. ability to engage in constructive diplomacy, because they have no confidence that diplomacy can be constructive, or because they see all diplomatic engagement as a snare to be avoided, or because they prefer heightened tensions and an increased likelihood of armed conflict. The hawkish loathing of diplomacy isn’t new or surprising, but it has found a new and unprecedented expression in the obnoxious letter to Iran’s government.