Economist David Tufte takes a victory lap to celebrate a prediction from 18 weeks ago come true. (Well, maybe only half true--we'll see.)
Support for (the first half of) Professor Tufte's theory here.
Another theory I read somewhere that I can't find again is that Democratic support for Biden was based on his role in advancing the Ukrainian impeachment baloney. (Trump was out to get a potential rival for the presidency!) Now that he is no longer needed for that the theory was that Biden's support would evaporate.