Honestly, I think Harris has consistently one of the candidates whose electability I'm most worried about. Who I think is most and least electable has fluctuated a lot, but Harris has always been near the bottom, but still above Warren, who I'm convinced
will lose. I'm still worried about her electability, but reactions to the debate reinforced concerns I had about several other candidates, so she's now in the middle of the pack in how electable I think candidates are, but because I think she's more electable, but simply because I think several other candidates are less so.
After the debate, I definitely thinks she's more electable than Biden, so seeing her gain at his expense is a net gain, even if I don't particularly like her or her electability. Harris was also the person I would have given a 40% chance of becoming the nominee two years ago, so I'm not really surprised that she basically won the debate.
Castro has always been a candidate I liked relatively well, but his debate performance has caused my opinion of him and his electability to diverge. On on hand, I like him more, but I'm somehow kind of getting Romneyesque vibes from him. And that, combined with his inability to translate favorability into first choice preferences has me worried that, if he wins, it will be as a compromise candidate because he's everybody's second choice, and that might result in an enthusiasm gap in the general.
ETA: FiveThirtyEight
has a new article where they discuss their data explicitly, and some data not from their polling. The big surprise is in gains in Twitter followers. Harris, Castro, Buttigieg, Yang, Williamson, Warren, and Gabbard gained the most in Twitter followers, in that order. Based on FiveThirtyEight's polling, Buttigieg's performance was average or a little below, and Yang and Williamson's performance were way below average.
I'd also wondered what the overlap between Buttigieg and Warren was. The article points out that they're the only two major candidates who did not mention Trump once. So the similarity in appeal seems to be "let's focus on America's problems and not the clown in the White House; we saw how making the election about Trump worked for Hillary." Given that re-elections tend to be referendums in incumbents, and Hillary's loss was arguably in part because Comey's letter made the election about her, I'm not sure that's necessarily the right takeaway, but I certainly don't object to the strategy. Especially not for Warren. If she's the nominee and manages to keep this strategy up through the general election, I think it makes her look less like Trump is living in her head, and that means she might not actually be guaranteed to lose.