Senator John McCain, whom you may remember from that one time he nearly became the president of the United States, has a less-than-satisfactory explanation for why he signed Senator Tom Cotton's open letter to Iranian leaders.

McCain told Politico that:

"I saw the letter, I saw that it looked reasonable to me and I signed it, that’s all. I sign lots of letters."

McCain's statement, reported late on Tuesday when the furor over Cotton's letter had become a national scandal, seems clearly intended to minimize his actions by making them seem commonplace and inconsequential.

Just signing a letter! Who among us hasn't signed letters? Basically the same as the family update your mom mails to all her cousins every Christmas — they're all just "letters."

Okay, Senator, you "sign lots of letters." But this letter was pretty clearly intended to undermine President Obama's position in the ongoing nuclear negotiations with Iran, by explicitly saying he would not be able to keep the promises he was making. This letter was addressed to the leadership of a hostile nation that is currently engaged in sensitive diplomatic negotiations with the United States, with the goal of undercutting the president of the United States in those negotiations.

Seems maybe worth pausing over a bit longer than the birthday card to a supporter's spouse.

In many ways, McCain's decision to sign the letter is more disturbing if he thinks it was merely a minor act. It's one thing to decide to actively and publicly undermine the president's conduct of foreign affairs, not just in this treaty negotiation but potentially in all other future negotiations, with all other countries, who will now also be able to point to this same letter as evidence that the president cannot be trusted to negotiate agreements on behalf of the United States.

But at least take that seriously. At least treat it as a weighty decision that carries significant, far-reaching consequences. Don't act like it's just more boring paperwork!