It’s about power:

Any argument about Dem/liberal behavior driving more people to the right founders on the reality that more people *aren’t* moving right

But in the US system, people who dislike the left have power disproportionate to population, and Dems haven’t figured out how to deal with that

— Alex Burns (@alexanderburns) May 13, 2018

That said, at least one Democrat has thought about this: David Faris. His book, It’s Time to Fight Dirty, describes how to do that (I’ve read the book; it’s a very good read). A key point is that Democrats have to stop worrying about norms, and, instead, exercise power so that power reflects the population.