With Trump in office, liberals seemed to consolidate behind a unified anti-Trump agenda. In a way, it radicalized them. The important thing was to be against Trump, therefore any nut who was against Trump was acceptable.
Here are two possible theories about a Biden administration.
1. The radical elements of the left — that were tolerated because they were anti-Trump — have become mainstream (for Democrats), and Biden will be captive to them. They have the energy and the enthusiasm.
2. Now that Trump is gone, more moderate Democratic elements can reassert themselves and cast off the nutty fringe. The “deep state” on the left will beat back the newcomers.
I hope for #2, and fear #1. But I don’t know which one to expect.