Biden needs to win back some of the states Hillary lost in 2016 like Michigan, Pennsylvania, Wisconsin, and to a lesser extent West Virginia. All states Trump turned red in 2016.
Florida is always in play, and interestingly because Trump won all of the states I mentioned above, he still could have lost Florida but still ended up above 270 electoral votes. But being that Florida is 29 electoral votes, a win there for Biden along with a win in either Wisconsin, Michigan, or Pennsylvania and Biden gets over 270 for a victory. Without Florida Biden has to win all 3 of those states. That is why Florida is more important to this election than California. Biden will get California anyway, he doesn't even need to campaign in that state other than to possibly prevent any congressional districts from slipping away from the dems. So in terms of Harris vs Demings, if the plan is to win the Presidency then he should want Deming as VP. But if it about defending down balloting only and preserving the HofR for dems---- then maybe Harris helps with that, but I don't agree.
Well for one she really isn't traditionally looked at by Black voters as really black. Demings does not have that problem. Harris is also pretty shrill by some people's standards, and she certainly was all of that during the debates when she forcefully attacked Joe Biden. Something which if Harris becomes the choice will have a lot of Republican political ads playing 24/7--- Harris attacking Biden.
Harris did really badly too in her run last year for the nomination. It was pretty flat, which is what I alluded to about her being polarizing. Is she popular to dems in California? Sure, but Biden already has all of those votes. If Biden picked Harris I would be stunned. And to add to this I think Harris has a problem with mid western middle America voters, especially gun owning democrat hunters and such. Harris' anti gun rhetoric is pretty big.... again, something polarizing when the plan is to win back some of those fly over / rust belt states so needed by the dems this year.