Not the funniest, but whatever:
If in 2016/2020 the Republicans nominate a Black guy and the Democrats a white guy then maybe.
Yeah, because there are a lot of black people in Washington.
As of 2007 the break down was
White- 36.3%
Black- 55.6%
Asian- 3.1%
Native American- 0.2%
Other- 4.8%
Hispanic (of any race)- 8.3%
As you can see, the majority of Washington D.C residents are black. Obviously, it isn't just the black people that vote Democratic, the suburbs of the nations capital tend to be very liberal. However, race politics tend to be more important amongst African Americans then the various white ethnicities, as can be demostrated by Obama's lock on the African American vote, seen in the flipping of North Carolina in 2008. Thus, if a sufficiently moderate African American can be found by the Republic Party (i.e. Colin Powell) and his opponent was a white Democrat, then in this situation I could plausibly see Washington D.C going for the GOP.
This is about the state of Washington, which is over 80% white and only about 4% black.