He's hoping to win by invalidating her. Problem is, this is not a primary and she's not a republican. He's gonna have to debate her eventually if he wants to knock her stride, because she's not going to do any interviews with anyone that's going to seriously attack her. She has all the luster right now, and as much as I have said Trump was probably going to win in November, Biden actually dropping out and Kamala stepping up was never part of my calculus.
For what it's worth, I don't want him to debate her either. Makes her road to the presidency easier.