Trump in a landslide if Harris is the nominee. She's viewed as insufferable and isn't going to change her low favorability rating in such a short time.
Well...If there's to be a switch, then it must be planned, crafted, and reasoned, and with full support of the party (or as full as it gets).
The first thing is for Biden to pull out. Hopefully by Monday, but no later than Wednesday. They'll strategize over the weekend no doubt.
A few more days is less time on the clock, but far better than a ham-handed announcement that reeks of desperation.