2020's double wave
With literally millions of votes still to be counted, Joe Biden's popular vote total stands at 73 million. That's already more than 7 million more votes than Hillary Clinton received in 2016 and by far the most for a presidential candidate of either party. There is no doubt that this election saw a massive, historic wave of voter turnout for Democrats.
The problem for them is it was a giant (albeit slightly smaller) wave election for President Trump, too. This election will also see the largest total turnout in American history.
Many Democrats are disappointed that the party lost some House seats and didn't make the big gains in the Senate they had hoped for, causing them to re-examine their strategies. But it is worth highlighting that their problem is less their failures to get out votes and more Trump's successes.
For example, in Texas Biden got 5.2 million votes, half a million more votes than Trump won the state with in 2016. In Florida, Biden got 5.3 million votes, roughly 700,000 more votes than Trump won the state with last time. These would have been considered great numbers, except that Trump managed to improve his vote total in both states by a million each. All together, with millions of votes left to count, Trump has managed to win six million more votes than he did last time, and will end up with by far the second most total votes for president ever (behind Biden).
A similar story took place down-ballot. In Georgia, Democrat John Ossoff trails in a race that will likely head to a runoff after receiving 2.3 million votes. But in 2016, Republican Johnny Isakson easily won a Senate seat there with just 2.1 million votes. The same goes for North Carolina, where the Democratic Senate candidate, Cal Cunningham, is behind but got hundreds of thousands more votes than the state's 2016 winner.
If two months ago you had told Democrats the numbers they would get in many of these races, they would likely have assumed landslide victories. Similar turnout percentages among Democrats in basically any recent election would have produced massive seat gains. We didn't see that this time because, while Trump rallied people to turn out against him, he also convinced millions of first-time or infrequent voters to turn out for him.
The 2018 midterms also support this idea. That year, opposition to Trump drove enormous Democratic turnout, boosting the overall numbers to record levels. But since Trump himself wasn't on the ballot, there was no similar increase among Republican voters, and Democrats won big all over the map. Many assumed we would see something similar this year, but the big surprise is that Trump was uniquely able to bring many new people out to vote for him specifically.
While Democrats should certainly spend some time trying to understand what they could have done better in this campaign, it is important not to lose sight of the fact they never before came close to turning out as many supporters as they did this time. It is tough to imagine what policies or actions could have driven turnout on their side much higher than opposition to Trump.