The underlying grievance is of course not over "increased diversity" per se but rather the long-term decline in civic attitudes. The progressive left expresses the exact same concerns in a different wording, by decrying the increased "marginalization and exclusion" that's seemingly entailed by such diversity. Guess what: they're all looking at pretty much the same thing.

I fail to see how the previous US president's complaints about people coming from "s*hole countries" and the exhortations from the Charlottesville rioters about how "Jews will not replace us" are grievances about "decline in civic attitudes". Try as one may to dissociate them, but these are the faces, words, and animating motivation of nationalism today.

Your argument is intentionally mixing "is" with "was".

Marx died 140 years ago, and there are no ethno-nationalist voices of significance on the left today. In the US context, after the civil rights movement of the 20th century those people left the Democratic party as the parties realigned themselves.

Today it is largely a right wing phenomenon, carrying forward the tradition of right wing ethno-nationalism that persisted from the previous century.