I wanted to dig up how big the minority effect might be -- some good stats in this document:
http://www.nd.edu/~cwolbrec/PartyID.pdfwhile registered blacks are overwhelmingly democratic, (7% republican, 81% democrat) they only make up 16% of the national sample.
females (37% republican, 53% democrat) probably have a stronger effect.
these numbers are from 04, though, so might be a bit out of date.