You’d see the parties change views if the electoral system were more fair. Democrats would likely become more left wing and Republican politicians more moderate, at least on the national stage. The two party system would continue.
It wouldn’t, long term IMO. The parties would end up roughly with similar overall proportions. Their positions would change. Democrats, with greater electoral power, will focus on internal competition for who is more ideologically pure & thus experience a leftward shift, to the point of probable over-reach (while Republicans, losing a lot of their in-built electoral advantage, will focus more on electability and thus will moderate, making them more palatable for marginal/independent voters), which is the mechanism for restoring the balance of power.
Considering the degree to which America lags behind the rest of the developed world in things like medical access, vacation, worker protections, etc. That would probably be a good thing.
