The problem is, that reasoning only reinforces one of two possibilities.
1 - they aren't informed about racism enough, therefore they literally cannot identify racism until someone calls someone else the N-word or whatever - that means they're ignorant.
2 - they are aware, and they simply don't care. Therefore, the difference between them being racist, and them being accepting of racism isn't too dissimilar.
The reason I say this is because you can't disassociate Trump's rhetoric from racism. From the stereotyping, to the name calling, to the flat out lies. It goes hand in hand, and if the voter can't see this, its because they either agree, or they don't care.