Politics is a game of addition, not subtraction. Right now, Trump isn't adding. And he's currently not doing well at it.
Let say, for shitz and giggles, Kamala isnt black and she is Indian/Mexican/Caucasian/etc. What is it about that would make people on the left, undecideds, and independents that are voting for Kamala go "Oh they have a point. I'm switching my vote from Kamala to Trump!"?
"Well she's a phony! The whole thing is a scam!" Ok, lets say it is. Again, what about that that is going to make left, undecideds and independents have a wake up moment and go vote for Trump? Any takers wanna answer that?
Attacking a black woman and accusing her, very publicly, of not being black, WHO exactly are you adding to your voter base who wasn't already in that base?
Not every Trump supporter is a racist, but every racist in America is a Trump supporter.