Thats in an ideal world, but its not practical for the US in particular because their system only allows for 2 parties

I dont mean electoral force, I mean popular resistance.

Even 'working within internal party politics' involves the use of force, because capital is constantly doing the same.

"In reality", democrats are beholden to the same forces of capitalism as the Republicans are - pushing them 'to the left' will always involve a threat of force greater than the threat posed by capital.