As I’ve said elsewhere, to fix it from within the system itself you would need a bloc of people willing to punish one of the parties for moving right by withholding their votes and their donations. To fix it from without the system is also possible, but would require some “authoritarianism” in the form of people with guns. Anything else is just people flailing around ineffectually and getting mad at others who aren’t flailing in the ineffectual way that they prefer.

Either way you’ve got the same problem: getting a large enough group of people together who are willing to do the thing. Socialists of all stripes have been trying to crack that egg for over a hundred years and the only ones who had any success were the ones that managed to get the peasants on their side because peasants have a certain amount of class consciousness that proles don’t.