It’s a problem inherent to big tent politics. A big tent party has to be all things to all people, and the US traditionally had two, The GOP seems to have solved this by becoming an almost exclusively fascist and reactionary party. The mainstream Democrats have responded by moving to the right but this alienates the left wing of the party, and they are now angry and mobilising.
The UK on the other hand doesn’t really do big tent politics, any party that starts that way will get pulled in one direction or the other.
Take me as an example. I am a Communalist. I know for a fact that the Labour Party will not let me be a member even with Corbyn as leader, I doubt that the Lib Dems would let me join and I am so opposed to the Tories that I don’t think they would even send me a form. The Green Party are the closest to my beliefs, and they might let me join, but I would still be on the libertarian left fringe of the party and I don’t think they would be that happy with the concept of counterpower.
The Democrats on the other hand would allow me to join, even though the members on the right of the party are more like British conservatives
