The recent letter of 47 senators to the government of Iran was rude, ill-considered, and largely detrimental to the country. However, it's recently been brought up that it may violate the Logan Act, a 200+ year old law which has never been the basis of a prosecution.
The only purpose of such a law is as a political club with which to threaten those who don't believe in the same policies. It may not be Constitutional under the first amendment, will clearly never be used, and if it were used would only deepen serious rifts in or politics.
Doing this now also may be something that the government can actually get done, given that the Republicans would probably be in the mood to support it at the moment and wise Democrats can probably see the wisdom in repeal.