One story I heard on cable is that Corker (Tenn.) and some of the others opposed the letter because they want to get some Senate Dems to sign on to a bill negating this agreement to try to over-ride a Obama veto, that the letter just gets Senate Dems to all side with Obama in which case a veto goes unchallenged .
Just a theory.
I never bought that override theory. The democrats care more about protecting Obama than getting their bill passed.
In my view, it’s just another trick by a moderate republican (allied with the democrats) to convince conservatives to not EVER rock any boat.