There is a good book on this called "Uncivil Agreement" which I enjoyed. It covers how, on both sides of the US political spectrum, identity trumps political issues in what people care about.

I think identity has been more of a focus of right-wing movements right throughout history, although people frequently refer to it in the context of the current US progressive zeitgeist; identity and nationalism are very intertwined. Identity and what it means to be an American plays a large role in Trump's support, for instance.