If you say it rationally, it's a rational act. The belief that he's a dumbass is what's irrational.
As Joe Biden would say, "look", he may do things that you don't like, that you certainly don't agree with, but that doesn't make him stupid, or to use your term, a dumbass.
Under-estimating him, and his intelligence on the part of your kind and your DNC overlords, is a big part of what got him elected in 2016.
Play the game the same way in 2020 and expect a different result at your own peril.