If Mr. Obama wanted the Iran agreement to be binding on the United States he should have submitted it to the Senate for ratification as a treaty. Without ratification by the Senate, it was simply an understanding between Mr. Obama and the other signatories. It was not binding on future presidents.
The former constitutional law professor needs to study up on Article II, Section 2, Clause 2: “He [the President] shall have Power, by and with the Advice and Consent of the Senate, to make Treaties, provided two thirds of the Senators present concur . . .”
He should thank Trump, it was rather like taking off an unsightly lump, getting rid of that treaty.
Now he’s less likely to get stick about it in the future since it can’t become the epic fail that it would have been left untouched.