If I had had a long public life filled with actions whose only explanation are malice or irrationality, I don't think it would be slander, no, even if the argument wasn't very well constructed. That doesn't make it a good argument, I just don't think it qualifies as slander. (Or libel.) For one thing, slander or libel in the US has to be actually false.
I'll grant assumption, but flawed? What else is there? I recognize that there are some people who will explain his actions as "brilliant, wonderful, generous, pure of heart, etc." but I have no qualms simply calling them wrong.
