alt Hacker NewsYou're kinda correct but making the mistake that the democratic party is a uni-party and not a vibrant political group made up of hundreds of different vying interests.
There is only one group to blame and it's the same group that has controlled the party since the 90s: neoliberals. Neoliberals do not have a vision of the future worth fighting for, more corporate control, so it's not hard to see when other opposing forces can simply offer something different.
It's the same problem we saw in the 1920s that lead to growing fascism: pushing for austerity, more corporate control over society, and deregulation are all things that help fascists and authoritarians thrive.
I agree neoliberals are a very big part of the problem (and giving them the finger was one of Trump's trump cards, pardon the pun), but I don't think it stops there.
Another aspect, which applies to neoliberals and many other groups in the Democratic coalition, is the seeming abandonment the idea of representing voters (in a district) in favor of persuading/lecturing them. That really appeals to the true-believer partisans (who are getting represented), but alienates everyone else.
I'm pretty sure the Democrats would have at least 5 more Senate seats if they consistently ran candidates that focused on representing the state they're running in as a whole (and continued to do that after the election was over).