I agree with your thinking that the polls from before Biden dropped out that asked about Harris as a hypothetical are suspect because people think about hypotheticals differently.
Harris' #'s are lower on both favorable AND unfavorable because she has higher undecided/unknown #'s. As I see it that gives her both a higher ceiling AND a lower floor than Biden because as she becomes more familiar those undecided/unknown will move to either favorable or unfavorable.
Fundamentally this is a good thing for D's because they were losing before so the downside risk is minimal.
But I also think there's another dynamic. The instant a candidate you didn't want becomes your party's candidate, you're forced into a state of cognitive dissonance.
In this case, for people on the left that view her negatively, it would be trying to reconcile two things:
- I don't particularly like Kamala Harris (for whatever reason).
- I identify as a Democrat and need to support the Democrat Party nominee for President.
I would argue that one of those things will mentally break. People don't like Kamala Harris. They'll either break with the party or they'll suddenly find a way to tell themselves that Kamala Harris is a lot better than they thought she was.
I'd further argue that the latter happens a LOT more than the former. In a mostly binary political world, they look over at the Republican Party led by Trump and they know they CANNOT identify with that, so they have to make peace with Kamala rather than change their identity. They may not like it; they'll still do it.
The former is what happened to Trump with the never-Trumper movement. People who were Republicans, many of them lifelong and prominent, said "If backing the party means backing Trump, I'm out." I think most would still
say they're Republicans, because identity is sticky, but they're sitting there harkening to return to some pre-Trump version of the Republican Party that certainly doesn't exist now, and may never again. They haven't turned Democrat; they've become politically homeless.
The difficult bit for independents who view her negatively is that they face a different sort of cognitive dissonance:
- I don't like Trump and don't want him to be our President.
- I don't like Kamala and don't want her to be our President.
One way out of that is apathy and not voting. Another is to allow whichever one you hate more to determine your thought and I'd argue that if you do that the opposite one will naturally start looking better and better, until the cognitive dissonance breaks and you say "well X isn't so bad, I'm sure they'll do SOME things I like, and it's better than Y after all!" and you'll start feeling better about voting for X.
That's just my thought on the psychology angle behind it.