Before leaving the house, I find it helpful to imagine the median intelligence level in my geographic area (local, or country as a whole) and then recall that about half the population has a level below that point.
The issue isn't intelligence per se. It's ignorance (often willful ignorance), dogmatism, media illiteracy, political illiteracy, etc. There are many intelligent (but evil) people in the Trump administration and not every Trump voter is a dunce. Framing them all as stupid isn't useful, because it doesn't help us understand and counteract what's happening.
For many Americans (on both sides) politics is not about policy.
It's about tribalism and nihilism. Decades of political disfunction (defined by the failure of elected leaders to enact policy broadly supported by voters) has lead to a loss of faith in the ability of government (as currently structured) to deliver anything. If government (and other institutions) have failed to deliver anything to someone, it's understand why they may not care about its destruction.
Do not underestimate latent European fascism, which is easily promoted using the same issue: immigrants. Most countries have a not-quite-majority for doing their own version of Ice, including shooting civilians.
I keep reading in the news that Trump now polls the lowest ever for any US president, then I check Nate Silver and he's been hovering around the same value for the last 90 days or so.