The inverse is happening in the urban white North.
Unless there has been a rapid population shift in the cities, it doesn't really look like much of a shift. Here is the Dem share in presidential voting in some "core" urban counties (are you claiming they are getting increasingly black so fast?)
Philadelphia, PA
1980 58.66%
1984 64.94%
1988 66.60%
1992 68.16%
1996 77.44%
2000 80.04%
2004 80.44%
2008 83.01%
New York, NY
1980 62.40%
1984 72.06%
1988 76.14%
1992 78.20%
1996 79.96%
2000 79.76%
2004 82.08%
2008 85.70%
Suffolk, MA
1980 52.46%
1984 62.27%
1988 64.02%
1992 60.62%
1996 73.01%
2000 71.38%
2004 75.88%
2008 77.49%
Suffolk county (Boston and a couple of suburubs) was 58% white in 2000, BTW. Doesn't seem like much of a shift to Republicans to me.
Poll
