Well here's a shot. Washington DC's population has actually started growing again and its largest single religious denomination is Islam [1]. So if the Republicans moderate somewhat from the Tea Party-ism and begins winning the Muslim vote [2] again and as US blacks become more middle-class there may be a remote possibility that the Republicans win DC.
[1] This is perhaps a bit unfair as Roman Catholics, Presbyterians, Baptists etc. are counted separately by the estimates while all Muslims are counted together.
[2] Not a crackpot idea actually. The Muslims are a natural Republican constituency much like East Asians-well-off, socially conservative, law-abiding. If not for the increasingly anti-Muslim rhetoric by the party... 
Muslims in DC are Nation of Islam blacks, not the Middle Eastern immigrants you'd find in Astoria or Dearborn who used to vote Republican.
Also, DC is only 10% Muslim. Doubt it is the largest single denomination; at least Baptists must be more numerous, and probably Catholics, too.
