But one of the main "in-groups" of the Trump supporter base were the working class. In its current form "diversity" is nothing more than a weapon for classism.

We are completely fine understanding root causes and bias which may underlie racial or gender based observations, but conveniently turn a blind eye to well-studied drivers for phenomena like nativism.

A lot of the current form of diversity against the disenfranchised "in-group" amounts to "you're f'd for being poor and FU again for being unhappy about being poor".

It's acceptable (if short-sighted) to exclude people for being racist. It's not acceptable to exclude people for being poor and uneducated. The Democrats messed up by ignoring the pain of poor people, not by being insufficiently tolerant of racism.

Those principles might make sense in isolation, but we know that being poor and uneducated leads to being racist.

The fact that Sanders was predicted to do significantly better than Clinton against Trump means that there's plenty of votes to be won by listening to poor people without being racist.

And right now working class, rural, and blue-collar Americans are in the left-wing out-group.