For a history of the process in the US I recommend Ezra Klein's 2020 book: * https://en.wikipedia.org/wiki/Why_We%27re_Polarized https://en.wikipedia.org/wiki/

For a history of the process in the US I recommend Ezra Klein's 2020 book: * https://en.wikipedia.org/wiki/Why_We%27re_Polarized https://en.wikipedia.org/wiki/Why_We%27re_Polarized While the public tended to have particular leanings, the Democrats and the GOP didn't really bother 'sorting' themselves until the Southern Strategy where the GOP went after the southern Dixiecrats: * https://en.wikipedia.org/wiki/Southern_strategy https://en.wikipedia.org/wiki/Southern_strategy * https://en.wikipedia.org/wiki/Dixiecrat https://en.wikipedia.org/wiki/Dixiecrat It's worth noting that there's decent evidence that one's political leanings may also influenced by brain structure: * https://en.wikipedia.org/wiki/Biology_and_political_orientation https://en.wikipedia.org/wiki/Biology_and_political_orientat... * https://www.goodreads.com/book/show/853648.The_Political_Brain https://www.goodreads.com/book/show/853648.The_Political_Bra... * https://en.wikipedia.org/wiki/Moral_Politics_(book) https://en.wikipedia.org/wiki/Moral_Politics_(book)