I am a Democrat and will not be voting for Trump in the GE but I understand his appeal. He makes a few good points and there is a meta-narrative in his discourse that makes some kind of sense. Basically, he stands up for the little guy, a constituency Republicans have forgotten about. Free trade has hurt American workers, regardless of any other benefits it may bring. He acknowledges that and claims he will create trade deals that stop exporting jobs. He supports improved medical care for veterans (who are mostly of the little guy class). He supports immigration reform, mainly as a way to keep immigrants from competing for "American" jobs. He favors no taxes below annual incomes of $50 K for families. So it's something and whether you agree or not, it's clear how the appeal works.

There's also the incendiary, divisive rhetoric. It's not nice but it works, I believe, for two reasons.

First, there is certainly a white American resentment for Mexicans, the Chinese, Muslims, etc, especially if you are on the lower rungs economically. Obviously, there's an ugly racist element at work here. But he does the same thing, even more so, to other candidates, politicians in general, reporters, etc. No one has a lot of sympathy for those people to begin with but if you are a guy who feels screwed by "the system", personally attacking its members is just another plus.

Second, and I think more importantly, this type of rhetoric establishes Trump as his own guy. He's not a politician and says whatever he wants. People like that and it's refreshing even if you disagree with it, as I do. His views of political correctness btw, which has entrenched itself like terminal cancer in the U.S., are right on the mark.