The agreement was signed on behalf of their caucuses, it wasn’t just an gentleman’s agreement between the two of them. If the leaders of two countries sign an agreement on behalf of their countries, does that agreement become invalid if one of the countries selects a new leader? Does NAFTA become invalid if Trump loses the election. When you sign something on behalf of someone else, those aren’t just empty words. Unless you are a politician.
NAFTA is a binding agreement. This most certainly was not.