Trump may or may not be a racist (most Dems are, so he most likely is, and has made some disparaging comments in the past).... BUT, what he has been doing is using the Dem stereotype of Conservatives as racists to gin up the small faction of the right who are racists, and most likely to get the support of the Dem party members who are also racist.
It is the Dem party with the history of racism, and as a card carrying member of the left, he fits right in.