This is a really contrived hypothesis and probably wishful thinking considering the current state of the world, but hear me out:
You know how the Overton window gets shifted? Rightwing voters grew a lot in the US and Democrats had to get more to the right to appeal to them and not keep losing.
We just need to force the reverse. If Democrats keep winning elections Republicans will be forced to put out a candidate that’s more palatable to leftists sooner or later. Someone who isn’t a literal movie villain. At that point, Democrats will lose their only selling point (being the alternative to Satan himself), and they will have to actually push for leftist policies to get people to vote.
This can only happen if Democrats win a lot of times in a row though. Even one Republican win will ensure them that they can keep pushing fascists and have a chance to win.