Here is a theory for you all to disregard: the current debate/rift/whatever between the the "old guard" democrats like Pelosi / Schumer and "progressives" like Omar/AOC reveals the reason why the Democratic Party lost middle America voters in 2016: their politics don't scale well beyond large cities.
The current Democratic coalition is between wealthy, mostly white liberals in big metro areas and poor, mostly non-white liberals also in big metro areas The former group is happy to finance expanded social services for the benefit (mostly) of the latter group, as long as they get cheap labor and free movement of capital in return.
This creates a society without a middle class: certain cities/industries are net-winners, these cities see extreme increase in cost of living as the wealthy flock to them, the wealthy raise taxes to pay for additional social programs, and the combination of high COL and taxes prevent working class from becoming middle class. Look no further than NYC, where wealthy liberals cram into a $2m 3 bedroom apartment but cannot send their kids to public school because they are so bad. However, this coalition more or less works at the city level, as the working poor are more than happy to have more government support at their disposal.
This coalition fails at the national level. The additional 5-10% taxes that rich liberals are willing to pay in property, city, or state income tax are not sufficient to pay for the things poor liberals want at the national level. As such, you have poor liberals - who have significant political power in the Democratic Party - demanding actual taxes on the wealthy (no more carried interest, etc). as well as additional taxes on middle Americans.
The end result is a radicalized sect of the Democratic Party - omar / AOC / etc. - who offer almost nothing to middle class Americans, and who have every incentive to take pot shots at the establishment to bolster their political standing.