One interesting thing about this election is the extent that the 2016 post-mortems and subsequent arguments for how Democrats should win--by basically everyone!--don't necessarily look great in retrospect.

There were basically two major diagnoses for Clinton's win--and two main arguments for how Dems should win going forward. Neither is how Biden pulled it off

One theory was that Trump won by flipping white, working class Obama voters, and therefore Dems needed to lure them back--maybe with a populist economic pitch.
I think that explanation for Trump's win was accurate,
but Biden had very, very limited success with Obama-Trump vote

A lot of the data preelection suggested Biden did have some success there, but it's really, really hard to see the case for it now. In fact, many Obama-->Trump counties swung even *more* toward Trump in WI, OH, IA, etc.

A second theory was that Trump won because of a bad progressive, youth, and nonwhite (but especially Black) turnout. To win, all Democrats needed was to recreate the Obama turnout, win some Jill Stein votes, etc.

The evidence never really matched this theory IMO, but it looks even worse today. The black share of the electorate did not increase, and quite possibly dropped. The voter file data we have so far suggests that the partisan turnout balance was unchanged or even *better* for Rs

And most of all, this theory assumed--implicitly--that all Dems needed to do was win 2016, as the president had maxed out his support, hadn't won any new converts, and couldn't compete in a higher turnout election. That assumption was wrong

Instead, Biden wound up winning in a way that I think many post-2016 post-mortems allowed as a possibility, but that I think had relatively few advocates: more-or-less run the Clinton playbook, but with a less polarizing candidate

As a result, Biden's gains came in many of the same places that Clinton surged in four years ago: traditionally Republican, well-educated suburbs, and often with limited coat-tails for downballot Democrats

Many of the challenges identified for Democrats in their 2016 post-mortem still hold today: the Trump-Obama vote; something less than the turnout they wanted/assumed; and now the added issue of relative weakness among nonwhite voters

This time though, I'm not sure the Democrats can blame themselves. This was a referendum on Trump, and even if Clinton/Dem weaknesses created the opening for him four years ago he's now cemented these as strengths of his own.