Bloomberg made the comment about candidate Bill de Blasio in an interview with New York magazine due on newsstands Monday. It appeared on the magazine's web site Saturday.
De Blasio is white, but he has been polling well among blacks since he began airing television ads featuring his interracial family. His wife is black and the couple has a 16-year-old son.
He has also criticized Bloomberg as not doing enough for the poor.
Asked to explain why that makes the campaign "racist," Bloomberg told the magazine, "It's comparable to me pointing out I'm Jewish in attracting the Jewish vote."
There was no immediate response from de Blasio.
