Douthat links to Nate Silver's analysis of exit poll data, which indicates that the incomes of Trump voters are not very different from the incomes of people who voted for other Republican candidates. Trump voters do have less education, but the difference seems to be purely one of education, not income (and therefore probably not class, although as I mentioned in my last post, few surveys now ask about occupation). Why would education be relevant to support for Trump? This is where my post from a few weeks ago comes in. There are a lot of issues on which many people take positions that are pretty much ignored among political elites. One of them is illegal immigration, on which a lot of people have supported proposals to deport everyone who's here without authorization or build a wall along the Mexican border, but until the rise of Trump, those weren't taken seriously by political elites. Another is the proposal to ban discrimination by appearance, which I discussed in the post from a few weeks ago.
What unites these issues, and a lot of others, is that there seems to be a lot of popular support for "there ought to be a law" policies: that people should be discouraged or forbidden to do "bad" things and encouraged or required to do "good" things. Moreover, this sentiment is often stronger among less educated people. That may be because education affects values--e. g., education is associated with more support for civil liberties. Another factor is that educated people may give more thought to how something would work in practice--e. g., would it really be possible to expel all illegal immigrants, or to enforce a ban on discrimination by appearance?
The gap between popular and elite opinions would seem to give a lot of opportunity to candidates who appealed to popular positions that don't get much elite support. But usually those candidates don't go far (e. g. Tom Tancredo in 2008). I think that's because people usually figure that the elites know what they are doing to some extent--if all "respectable" political figures condemn or dismiss a position, there's probably something wrong with it. But when confidence in the elites is very low, like now, that barrier is gone, and there is an opening for candidates who are outside the mainstream. And for the reasons mentioned in the previous paragraph, the non-mainstream positions may be particularly appealing to less educated voters.