I've said from the beginning and I stand by it, if he chooses Harris, it's advantage Trump. I think his team probably knows this also, they do their own polling.
The problem with Biden now, is that he cornered himself in this regard, he announced he would choose a female VP. This, in and of itself is going to be a weakness for him in 2020 now. Trump is going to tout the many females he has in his cabinet, but, he can say "I never assumed I was going to choose someone on their gender or any other feature. All I wanted was the most talented, competent who could get things done"
So, no matter WHO Biden chooses. As long as they are female, it looks like a pandering choice.