Aside from conservatism (duh), Alan Abramowitz says the strongest predictor of support for the Tea Party is racial hostility. Paul Krugman says he thinks Donald Trump supporters are basically just tea partiers. Put these together and you get this:
So maybe Trump’s base is angry, fairly affluent white racists — sort of like The Donald himself, only not as rich? And maybe they’re not being hoodwinked?
Now, you might ask why angry racists are busting out of the channels the GOP constructed to direct their rage. But there, surely, we have to take account of two things: the real changes in America, which is becoming more socially and culturally diverse, plus the Fox News effect, which has created an angry white guy feedback loop.
Maybe. Here's a data point in favor of Krugman's thesis: the rapturous response Trump gets whenever he says he has no time for political correctness. It was one of the biggest applause lines he got in Thursday's debate. And while there are legitimate complaints to be had about some of the more extreme versions of language policing, for most people their real issue with it is that it forbids them from delivering casual slurs—that everyone knows are true—about blacks or women or Muslims or gays or whatever. They've been doing it all their lives, and they think it's ridiculous that they have to watch themselves in public lest someone think they're racists. Trump appeals to that sentiment.
I should add that this is entirely consistent with the notion that Trump's strength comes fundamentally from his appeal to the conservative culture of grievance and resentment. After all, what are tea partiers so resentful of? Wall Street banks? Maybe, but they sure don't seem to favor any serious action to rein them in. Corrupt politicians? Could be, but they keep electing them to Congress even if they grumble about it. Middle-class wage stagnation? Probably, but it can't be too big a deal since they consistently vote for politicians who are dedicated to doing nothing about it.
At a gut level, the answer is that they think "normal" American culture is under attack. Straight, white, Christian men used to run this country and did a pretty good job of it. But now every minority group in the country wants a piece of the pie, and they all blame "white supremacy culture" or "rape culture" or "heteronormative culture" for their problems. And what's worse, no one is even allowed to tell the truth about what this really means. Mexicans come pouring across the border but you get in trouble for just plainly saying what everyone knows: most of them are criminals and should be sent back. Muslims blow up the World Trade Center, but woe betide anyone who makes the common sense observation that we should keep a close eye on mosques because most of them are terrorist breeding grounds. Blacks commit violent crimes at higher levels than whites, but we all have to pretend this is only because whites have been keeping them down for so long. And if you make a harmless joke about some woman having a great body? It's a compliment! But the feminazis will be all over you like bees in a hive.
This is what a lot of them resent. It's even understandable: everyone is uncomfortable being told that something they're used to doing is now considered insulting. Certainly Donald Trump understands it. When he says America no longer has the luxury of worrying about political correctness, his supporters couldn't agree more.