If you're a member of a formerly majority group who is disenfranchised by a crappy economy and shrinking middle class.... (in other words - you're white person who's parents may have had it better than you do) and you're mad as hell about it... and you want to blame people with skin that is shades of brown darker than yours... and you want to be able to "speak your mind" without repercussions (in other words say racists and misogynist things without being called out for it)... and you want to blame politicians who seem to have buggered everything up... and you're not educated enough to know any better... and you don't understand what the office of the presidency is REALLY about... . . . and there's a dude yelling: IT'S OKAY TO BE MAD! I'M MAD TOO! IT'S OKAY TO DEMEAN WOMEN! I'VE BEEN DOING IT FOR YEARS! IT'S OKAY TO HATE ALL "MEXICANS" (even though he means all people of latin decent, and likely all people of skin browner than his) I'VE GOT A GREAT PLAN I'VE GOT YOOGE PLANS. I'm not saying I like it... but there's a YOOGE population of people who are just. that. dumb.
In other words, wishful thinking. People in Russia like Vladimir Zhirinovsky for the same reason, even though most of the population (that I've spoken to about it) pegs him a clown. Just listen to this (not his most passionate speech, but oh dear, is it biased; LDPR is the Liberal-Democratic Party of Russia) - or read this from BrainyQuotes. People seem to think that Zhirik is only on the political stage to create an illusion of democracy, of that change is possible; personally, I don't see how he'd be running the country: he seems to have no idea how to do so!
As fun as it is to call the other side dumb, I don't think that's it. He appeals to angry working class white people. The Republicans have been misdirecting their anger at minorities for a long time to get their support, but they're more sophisticated now. They're not going to go to the left, they'll be expected to do too much self-flagellating, and they're not secure enough to shrug that off. Trump takes some traditional left-populist positions, opposition to trade deals being the big one, and he's not going to make them feel bad for America's history of racism and misogyny because he's still racist and misogynist. He's manipulating people just as transparently, he just has a more modern strategy for doing it.
Perhaps uneducated? Or willingly ignorant, take your pick.
Angry, frustrated, afraid, those work better. People voting for Trump are not dumb; I'm surrounded by them and don't fucking dare put up a Sanders decal unless I want rocks thrown through my windows. People outside of the big tech cities are watching amazing things happen somewhere else as their way of life is slipping away... and nobody seems to care or even give a shit as long as the Internet is working and the pizza is being delivered for the sports stuff this weekend. Trump himself will be a joke and I am not afraid of him. I'm scared and concerned about what friends of his will be running the FCC, SEC, DEA, FDA, EPA etc. That is where the real fear should be. Of all the candidates, the only one that I think gives a damn about anyone who really needs to be told "hey, it sucks now but I'm working on it, I got your back" is Sanders. And the neat thing to watch is that the more some of these diehards are hearing about him because they hate the Clintons, the more they are telling me that he makes sense and seems genuine.