Updated on March 9, 2015 at 4:55 p.m.

It used to be said that politics stopped at the water's edge. Increasingly, that doesn't seem to be the case.

The latest indication: a letter from 47 Republican senators, most of the GOP caucus, to the supreme leader of Iran. The letter, organized by Arkansas's Tom Cotton and first reported by Josh Rogin, notes that any deal President Obama makes with Iranian negotiators about nuclear enrichment is not a duly ratified treaty and could be reversed once Obama leaves office in less than two years. That comes a week after Israeli Prime Minister Benjamin Netanyahu spoke to a joint session of Congress—a speech arranged by House Speaker John Boehner without first consulting the White House.

"It has come to our attention while observing your nuclear negotiations with our government that you may not fully understand our constitutional system," the letter begins somewhat patronizingly, then explains the treaty-ratification process, ending:

What these two constitutional provisions mean is that we will consider any agreement regarding your nuclear-weapons program that is not approved by the Congress as nothing more than an executive agreement between President Obama and Ayatollah Khamenei. The next president could revoke such an executive agreement with the stroke of a pen and future Congresses could modify the terms of the agreement at any time.

The intent of the letter and speech invitation aren't really in dispute: Both are attempts to scuttle a deal that would slow or halt Iranian nuclear enrichment without completely dismantling the program. The letter is notable as a highly unusual instance of the Senate directly intervening in the conduct of foreign policy, and it in essence aligns most Republican senators with Iranian hardliners who don't want to see any deal.

For Obama's supporters, the letter is an appalling breach of protocol: a group of senators—led by a freshman with less than three years in Washington—opening up a separate, rival channel to Iran. To the signatories and their supporters, the letter is a simple and incontrovertible statement of fact on how treaties work (although Jack Goldsmith notes that the letter doesn't quite get its facts straight about the Constitution).

As you may have noticed, those aren't mutually exclusive interpretations. Analysts have been warning for months that by choosing to effectively box Congress out of the process—perhaps by necessity, since there seems to be no deal that both Iran and a majority of the Senate would accept—Obama was building a deal on unsteady ground. But it's also hard to think of an occasion of the legislature working to undermine the executive branch on a foreign-policy decision quite like this.

Seven Republican senators are not signatories to the letter, and it's an interesting list:

  • Bob Corker of Tennessee, who is the chair of the Senate Foreign Relations Committee. Corker is the sponsor of a bill that would increase sanctions on Iran only if Congress rejects a final agreement with Iran;
  • Lamar Alexander of Tennessee, a Washington veteran. His spokesman said the senator had expressed his view by agreeing to co-sponsor the Corker bill;
  • Dan Coats of Indiana, whose office declined to comment;
  • Susan Collins of Maine, a leading moderate Republican;
  • Lisa Murkowski of Alaska, who also sometimes bucks her party;
  • Thad Cochran of Mississippi; and
  • Jeff Flake of Arizona, whose office told me Flake also supports Corker's bill because he thinks the Senate ought to have a say in any deal but considered the letter "unnecessary."

Other than Coats and Flake, requests to discuss the letter from the other senators' officers were not answered by press time. I'll update if I receive more information.

In a curious convergence, the signatory senators' stance puts them essentially on the same page as many in Iran, who oppose any deal, believing that diplomacy with the U.S. is unwise and that any agreement would be a dangerous surrender of national sovereignty. The two sides have been in a peculiar sort of masque dance for months; when Iran hawks in the U.S. Senate pushed to impose new sanctions on Tehran, the conservative faction in Iran responded by drafting a bill to ramp up uranium enrichment. One key difference is that while the Senate would have a role in approving any official treaty and can try to thwart any deal Obama might make, it is theoretically only Ayatollah Ali Khamenei, the supreme leader, who has the final say on any deal from the Iranian side.

As the nuclear negotiations grind toward a March 24 deadline (which might be extended), it seems—as is often in the case in diplomacy—that any agreement would contain bitter provisions for both sides. But in reply to those who don't want a deal, from Netanyahu to Cotton and his clique, critics including my colleague Jeffrey Goldberg have argued that the hardliners don't have any good alternative beyond sanctions, which have not driven Iran to give up the nuclear program yet. It's one thing to demand a better deal; actually getting it is an entirely different question, and one this letter doesn't do much to answer.