A severely unanalyzed factor: Bloomberg. Yes there were bumps but Bernie had started contrasting more with Biden and it was working. But then came Bloomberg's insane ad spending, and his attacks on Bernie, which meant that for 2 crucial weeks, Bloomberg was the focus.1/2 https://twitter.com/davidsirota/status/1250511694834491392

That not only took heat off Biden, it allowed Biden to reposition himself as the centre of the field rather than the right (which he is). In other words Bloomberg's investment paid off on the front he cared about most: stopping Bernie. We were up against a hydra-headed beast. 2/2