I think you don't understand the economic history of the United States. What you consider far left was the reason that the USA became so successful. I suggest you google the history of progressivism in the USA.

The reason the 50s and 60s were so successful (and which the Republicans want to go back to) was precisely because the country taxed the wealthy 90%, kept college cheap, had a lot of well paid jobs for college kids, etc. The economic situation was different precisely because the economic system was 'mixed.'

Tribalism increases when people feel economically threatened. So you're not going to get rid of tribalism until you fix the economic system, and you're not going to fix the economic system until you go back to the economic system that reined in America between the end of the 19th century and the 60s.