By far the biggest thing about voting is that it is very tribal. The easiest predictor of how anyone will vote is to identify the tribe they voted for last time. For most people that changes at most ONE time in their entire lifetime. Now of course it is a feedback loop–the parties try somewhat to cater to their regular voters. Though they only do so insofar as the party perceives that their votes might have a chance to go to the other side.
When things change, they typically change on the margin of that. MOST people still vote for whomever they voted for last time. But JUST ENOUGH people change that behavior to swing the election.
So when it comes to the working class, it is easy to predict that on a general basis Democrats will have won more of the working class. They did so last election, two elections ago, and three.
Clinton appears to have lost because of two things–getting MANY fewer of traditional Democratic voters to vote for her at all nationwide (including in Democratic stronghold states where voter suppression can’t be a serious enough issue to explain the drop), and showing a very large swing (in the context of the fact that people almost never change sides) against her regarding the working class in the Rust Belt States.
The current meme explaining that among Democrats appears to be ‘racism’. That is a highly problematic claim in general because it doesn’t appear to show the change in voting patterns. (Most evidence for this takes the form of “the Republicans harbor racists”. But the actual racists have been voting for Republicans for at least 4-6 presidential cycles so that doesn’t explain the CHANGE).
The Rust Belt States especially show the problem with this explanation: they went for Obama (an actual black man) twice by relatively comfortable margins. For the most part, we fully expect that they will stick with whomever they voted for before–because almost everyone does that. But quite a few of them went from twice voting for a black man who was running as a Democrat for President, to voting for a Republican.
You have to thread the racism needle pretty tight to allow for a big swing of people who were willing to vote for Democrats most of their adult lives, who were willing to vote for a Democratic black candidate for president, and then who suddenly went racist enough to vote for a Republican.
It seems much more intuitive that they changed their minds about something other than race.
Now a much more valid analysis might be that they weren’t put off by Trump’s racism nearly as much as we wish they would have been. They let their personal concerns about job stagnation and job insecurity swamp concerns about Trump’s signaled racism. But that is a VERY different argument from the “they are just racists and I won’t compromise on racism” argument.
POSTSCRIPT: We still need to do whatever we can to fight the actual racists who have been emboldened by the way they their association with Trump is validated by his election. We just don't need to overreact by writing off the huge segment of people who are normally open to us.