2020 Campaign Crowd Size
While Crowd Size vs. Vote Count doesn't prove anything, it hints strongly. Facebook and Twitter try to suppress this with fake Fact Checks saying that the reason Biden couldn't draw crowds was only because he didn't want to and was "social distancing", but he did try a few times, and fell flat. Those campaigning with/for him tried, and fell flat. Polls showed a HUGE voter enthusiasm difference, and that translates to votes. So why did Biden outperform Obama and everyone in history, as such a hugely flawed candidate.
There is an argument to be made that it was because the left so despised Trump that they turned out just to vote against him. And while the left being hateful and spiteful and only doing their civic responsibility to screw the other guy fits their culture -- it's still highly improbable. The polls did not really show that degree of animosity, with Trump often out-polling Obama, and that's with the huge Shy Trump Voter effect in play. So you have to have no critical thinking or skepticism to believe this fully explains the effect. Especially in light of all the other things that are additive towards a corrupted election.