This is a tough nut to crack. I think Trump's support amongst the non-rich is actually varied. In the mid-west, I think his support boiled down to either racism (which he was very open about) and his anti-NAFTA, pro-US labor rhetoric, particularly when you consider his primary and general election opponents.
But I think at some level whether it's racism, economic resentment towards the coasts, a general sentiment that whites are worse off than their parents are, or all of the other possible reasons --I think the average American understands that the country is currently headed in the wrong direction. I think the people who support Trump have correctly identified that there's something deeply wrong with the way the US comports itself and how our economic system is arranged, but they don't really have a fine-grain understanding of why they are getting so screwed by the system. The the far right-wing media has offered them a lot of different scapegoats --"lazy black people", the mooching poor, Jews, the "Wall Street city liberals", government intervention-- and largely Fox News has done nothing but further encourage this brand of bull****.
No matter what the specifics are though, Trump did something that no other candidate (other than Bernie Sanders) did:
1.) Acknowledged that there was a problem.
2.) Gave his honest opinions (however uninformed or malleable) about what he thought they were.
3.) Did not present himself as a traditional politician, which gave credence to his claim that he knew there was a problem. He also pointed out that politicians around him are corrupt during the primary.
4.) Gave people whatever answer they wanted to hear about the problem. This made them start believing that he was a sensible, even if amateurish, politician.
This is fundamentally an emotional argument. This was all to get people to buy into his brand, his image. Now that he isn't delivering these goods, it's sort of immaterial. Until there's a competitor politician who can actually speak to people's problems and thus address their core emotional reason for supporting Trump, Trump still holds a monopoly in the emotional political sphere. Many of his supporters will still feel like even discussing facts is a non-starter, because if you can't even acknowledge that America as it exists right now is a problem, and honestly tell people that (for most Americans) they are worse off than their parents --then the core anxieties that they are experiencing aren't being addressed by "Don't you realize that Trump is lying when he says X?"-style arguments. In fact, those arguments on their face are not appealing to them. I think this is one of the things that liberals (particularly and especially upper-class liberals on the coasts) both badly understand and they are ill-equipped to understand.
It's also worth adding that for some contingent of poor whites, this is almost entirely racial, and their anxieties are rooted in the idea that they are experiencing black people not being smashed into the dirt and that upsets them. But that is very far from the only thing driving these people's anxieties (and by extension, Trump's approval ratings). So when the time comes to figure out how to get support and how to change hearts-and-minds, it'll matter that the non-racist whites who've been harmed by this system, who aren't racists, and who will be willing to work with disadvantaged groups can be swayed from Trump.
