With progress having been made on the Iran negotiations, Republican Senators opposed to a deal have been threatening the administration left and right over it. Today, they took a different tack, issuing an open letter to Iran, warning them against the deal on the grounds that they’re just going to sabotage it in the future.

The letter was pushed by Sen. Tom Cotton (R – AR) and signed by 47 senators. Surprisingly, this included Sen. Rand Paul (R – KY), who had previously expressed opposition to Congressional attempts to sabotage the negotiations.

Just a month ago, Sen. Paul had admonished the Senate against standing in the way of negotiations in good faith. Now, with Iran a key issue in the upcoming presidential primaries, he seems to be wavering on the matter, and towing the party line.

The letter provoked a sharp criticism from the White House over the Senate’s attempt to interfere in diplomatic efforts, and a dismissal from Iranian Foreign Minister Javad Zarif, who said Iran didn’t take the threat seriously.

Whether Zarif’s comments reflect the view of the entire Iranian government remains to be seen, however, and opposition from US hawks, and a threat to sabotage the deal, should only add to the calls from Iran’s own right-wing to ditch the negotiations on the grounds that the US can’t be trusted to keep its bargains.

The argument has been pushed for a long time by Iran’s Conservative parliamentarians, who have been averse to the deal. With US Republicans now confirming as much, and explicitly writing a letter saying they can’t be trusted to keep any deals reached by Obama, the talks will surely suffer at least somewhat.