American Carnage

American Carnage: On the Front Lines of the Republican Civil War and the Rise of President Trump (2020) by Tim Alberta is probably the most comprehensive explanation of the question "how did the Republican party turn into Trump's party" I have found.

It discusses the roots of the far right movements, from how Eisenhower dealt with the John Birch Society, through the Tea party becoming unmanageable for figures such as Paul Ryan and John Boehner, through John McCain's incompatibility with the extreme wing (leading to the selection of Sarah Palin for VP on the ticket) and on into today.

The book makes the compelling case that Trump (and Fox News, for that matter) are a product of these long term trends, not a cause. It's an excellent companion piece to The Authoritarians, and both should be considered when thinking about both US politics and the cultural gaps that exist today.