Forty-seven Republican Senators signed a letter to the Iranian leadership claiming that any nuclear non-proliferation treaty signed by President Obama could be vacated at the stroke of a pen by the next President. What is the impact of their tactic on the probability of the deal being signed?

There are many effects but here are four key ones:

1. The Iranians now get further confirmation that President Bush III will be tougher than President Obama. The deal on the table is the best they are going to get. This makes them more likely to sign.

2. Democrats who are skeptical of the deal with now rally round the President, as they did after the surprise Netanyahu speech. This makes a deal more likely.

3. The Iranians now get further confirmation that the deal may fall apart under next administration. This makes it easier for them to renege in the future if circumstances dictate – they can more credibly blame the U.S. for being untrustworthy. Their citizens will not blame them if they exit the agreement as the Iranian leadership can more credibly blame the U.S.

Russia and China can trade with Iran with less international condemnation as the U.S. can be more credibly faulted for the deal falling apart. This makes a deal more likely.

4. The “no deal” option just got better too for reasons outlined in 3. Iran can blame the U.S. for not agreeing to the treaty and try to persuade Russia etc to break sanctions. This makes “no deal” more attractive relative to “deal” and makes the deal less likely. It could also mean that the deal the Iranians now get is improved to reflect their better outside option. If the deal is already better than even the improved outside option, the improvement will have little effect on the chance of a deal.

So, unfortunately, the net effect of the letter on the probability of an agreement is ambiguous. But even if the letter makes a deal less likely, it makes it less likely by making Iran stronger.