"That’s a real problem for the Democrats, especially if all the racial politics they pursue in order to try to motivate minority voters (Black Lives Matter, immigration protests, etc.) actually serve to make minorities less likely to vote for whites, even if they’re Democrats. And if working-class whites start to vote Republican the way minorities have voted Democratic — and all that racial politics is likely to encourage that — the Dems are in trouble."
You can't control human minds. If you get people to think of one thing, they're not going to stay with that one thing. It will remind them of other things and there are consequences. It seems as though the Democratic Party had the idea of telling people in some groups that they should think of themselves as members of that group and to do what is advantageous to their group interests, which is to support the Democratic Party.
But the message doesn't stick and remain fixed. For one thing, everyone, not just the people in the targeted group, hears the message, and those non-targeted people may react, feeling perhaps that they should think of themselves as members of their group and to pursue the interests of their group.
And the people in the targeted groups might resist being grouped the way the Party wants them to stay grouped or they may accept the grouping but question whether the Party that wants to own that group really has its best interests at heart. After all, the Party has the open and obvious goal of getting power, and everyone can see that they are being used as a means to the Party's end.