What the senators did could technically count as treason. The good news is that Republicans in the Senate have noticed that their government is involved in a negotiation with Iran to prevent it acquiring a nuclear weapon. Well done, all. You’re right, it’s a big deal. But not for the petty partisan, power-envy reasons that have woken you to the issue. It was June 2006 – Margaret Beckett was Foreign Secretary – when all five permanent members of the UN Security Council, plus Germany, began what has been about as painstaking and delicate a diplomatic poker game as you are ever likely to see. And with proportionally high stakes for all of us. Yet it is now that they barge in with their “open letter” to Iran’s leadership.

“It has come to our attention…” the missive, dispatched this week, ludicrously begins before going on to warn that any deal that might emerge may simply be ripped up in barely two years when – they desperately hope – a Republican may have taken the White House. Senators imply they’re in the dark about what precisely is happening at the negotiating table in Geneva and can’t trust President Barack Obama and his Secretary of State, John Kerry, to deliver what they promise, to keep Iran from becoming a nuclear power. What they are really in the dark about is their own foolishness and how foreign diplomacy works. It doesn’t work by deliberately seeking to cut the legs from under your Commander-in-Chief when he seems within millimeters of an international arms control deal.

You might almost charge them with treason, under the provisions of the 1799 Logan Act that forbids any American from conferring with a foreign power with intent to “defeat the measures of the US”. No one is suggesting taking that route. But admire the restraint of Mr. Kerry, who recently found himself facing Senator Marco Rubio, a Florida Republican, possible presidential aspirant and signatory of the letter, at a hearing on war powers to tackle Isis. The deal on the table is as described, he said. It is not about making Tehran an ally in the fight against Isis or anything else. It’s about nukes.

“I think this has been a misread by a lot of people on the Hill to be honest with you,” Mr. Kerry said. “There is no grand bargain being discussed here in the context of this negotiation. This is about nuclear weapon potential. That’s it. It is really almost insulting that the presumption up here is that we are going to negotiate something that allows them to get a nuclear weapon.” Not that he was going to leave without directly mentioning the missive. “My reaction to the letter was utter disbelief,” he declared, adding that in the 25 years that he spent in the Senate he had “never heard of, or heard of being proposed, anything comparable”.

That’s because the partisan enmity here has never been so acrid. The Republicans now control 70 per cent of all state legislative chambers, the highest in the party’s history. They now rule Congress too. So they just can’t stand it that the White House remains beyond their reach and may elude them again in 2016. And when Mr. Obama tries to circumvent Congress, they go nuts. But when it comes to foreign policy, an American president, of either party, can, in some instances, expect exclusive authority. This is such a case. What the senators have overlooked is that the putative Iran deal will not be a treaty, which would require Congressional approval, but an executive agreement with a foreign state which would not. If concluded, it may also be codified into a UN Security Council resolution. The notion that an incoming Republican president would simply undo it is fanciful.

But this is far pettier in provenance. It’s about hurting Mr. Obama.