There are good progressives who sincerely believe the Democratic leadership/establishment is antithetical to true progressive change. They don't think you can defeat the GOP and their oligarch benefactors without transforming the Democratic Party.
...so they focus their fire on Dems even as Republicans wreak havoc. And there are progressives who vehemently disagree, arguing it's folly to attack Dems as the GOP lurches toward fascism. This is more a strategic than an ideological debate.
During my two decades in politics, I've done both, pushing progressive causes by critiquing the meekness of Dem leaders in the face of Republican authoritarianism, and focusing on the GOP's lies, hypocrisy, and fealty to the billionaire class.
There's nothing wrong with debating these approaches. The problem is that it manifests as disputes over particular politicians rather than issues or strategy. Individual pols (Hillary, Bernie, AOC, Feinstein, Pelosi, Schumer) become proxies...
When we become attached to people and infuse them with more power and importance than they have, we lose sight of the larger progressive cause that we all share. And there are hostile entities who exploit our sincere strategic disagreements...
So I hope that those fighting for fairness, equality and justice look past the attachments to certain political figures and focus on the larger progressive cause. 2020 is truly our last chance to stop the far right's takeover of our democracy.