Weird that the first post-debate ad buy from Donald Trump is a digital ad in the seven swing states simply saying that a vote for Biden is a vote for Harris (with no actual explanation for why that's bad).
There are a couple options here.
1. They are very bad at this and they're doing this because they're racist and they assume other people are.
2. They've done their own polling and found that Biden is more popular than Harris and so they want to make her the focus of the race.
Australian politics some 20 years ago. "A vote for Howard is a vote for Costello". It failed because if you were going to vote Howard, having a successor was a bonus, and if you weren't going to vote for him then it didn't matter. Of course sadly in this case skin colour and gender may be an issue.
She is a cop, and her tenure as the head of California's grotesque carceral state was notably exploitative.
But somehow I don't think that's what he's alluding to.
A lot of democrats already know this and they are hesitating. Nothing scares a considerable majority of white Americans, a majority of Democrats even, more than a Black woman in power.