Joe Biden was better rehearsed in his talking points for the second debate, but that's all he had: talking points, delivered with a singular absence of charisma. Primary voters who imagined him on a debate stage with Tr*mp much have felt more than a little queasy, even as they tried to figure out which of the other establishment white males were which.
Kamala Harris is trying to bridge the divide between the progressives and the moderates, but was visibly stressed during the second round of debates; and Pete Buttegeig's base of supporters seems to have topped out. Unless there's an unexpected surge by Amy Klobuchar or Tulsi Gabbard, Biden is likely to remain the anointed choice of "moderates" in the party's leadership — potentially with devastating results.
Here's my nightmare scenario: Democrats go to their Milwaukee convention next July with no clear leader. Hoping to maintain the Democratic advantage among women, Bernie Sanders releases his delegates to Elizabeth Warren, and it comes down to a choice between Warren and Biden. Elected delegates are split, and establishment super-delegates throw the nomination to Biden. On Election Day, large numbers of young progressives stay home in disgust: Tr*mp redux.
One last thought: in the September debate, I genuinely will miss Marianne Williamson — the only candidate to make a consistently moral argument against Tr*mpism. Hopefully, some others will be more inclined to call out evil when they see it.