His supporters don't care if Trump's fit to be President or not. All they care about is that he hits the same grievances and whinny complaints, and hates the same people, as they do.
Yes.
I don't like to say "whinny complaints" - but he's playing a great tune. A lot of people are unhappy with their lot in life. Aside from it being unpleasant to blame one's self for your lot in life, in a country as rich as this one, if you aren't participating, there probably is a thumb on the scale against you.
Trump - either through ignorance or malice - develops a narrative that hits.
You got laid off or not being paid a lot. There are X number of Mexicans who came to the US and are working. They must have taken your job. But the labor market for Mexicans crawling across the border and marginally skilled US native citizens with reasonable English skills doesn't have a lot of overlap. You aren't losing your job to a Mexican, the Mexican is keeping your consumer prices lower. But that conclusion doesn't fix your problem.
Some kids in your area died of fentanyl. Must be the Mexicans smuggling it across the border. In fact, if there is demand for drugs, that demand will be satiated, imported through the border or home grown. I saw multiple billboards on I-70 in Missouri - "Fentanyl - an unwanted party guest" warning of the dangers of questionable street drugs. We don't have those billboards here. Drug use is more prevalent when despair is more prevalent. Fix the despair, fix the drug use - you can't just try to put a finger in the fentanyl dike, it will flow in some other hole. But it's hard to create better conditions and probably requires some governmental help but if you've been told the government is the problem and that they are wasting your tax dollars, a lot easier to just blame Mexico and we can send mens with guns and save our kids.