People who believe in the Second Amendment, controlling our borders, law and order, fair trade deals, and not necessarily believe in choir long politicians are the underdog. He is horrible at it because it is horrible communication skills but he’s the only one speaking for those people.
When the other side "speaks for those people" in an obvious attempt to win their votes, we call it "pandering."
I don't see that Trump is doing anything different than that.
To the extent that he's been political, he was a lifelong centrist Democrat, perhaps because it would not have helped him to be a Republican in New York. He was friends with the Clintons.
When it was obvious that he had no career as a Democrat on the national stage, he switched and used the GOP instead.
He doesn't care about those little people whose voice has been ignored any more than any demagogue cares about the people to whom he appeals. Those people are just means to an end, like people in the military are and Big Ten football fans are. He doesn't care about them. He cares about himself.
If he were on the other "side," Republicans would be calling him out every day, just as they did with Obama, who was not as obvious a demagogue as Trump is.