The problem with calling them dumb is that it’s dismissive. It’s a permission to say that they’re wrong simply because their brain doesn’t work, but that’s not true at all. For every position that anyone holds, they’ve got reasons. They may be choosing reasons that you disagree with, or they may be misinformed on the facts. They could even just have an emotional connection and not have reasoned through it at all.
But in their fully functioning brain, they hold a position that’s important to them. If you can’t or won’t acknowledge that, you’ll never be able to change their mind or even meet in the middle.
I’m the farthest thing in the world from a Trump supporter, but his supporters have their reasons. He’s polling best among white non-college educated men, a group that has seen their jobs prospects and wages decline, their life expectancy drop and their automatically allotted share of the social contract diminish. Trump is giving them someone to blame and providing a national voice for their unhappiness. I think they’re being self centered, but they’re not just “dumb”.