adding it to Virginia I get:
Obama: 2,238,890 (54.0%)
Romney: 1,843,903 (44.5%)
Others: 65,460 (1.6%)
Compared to the original Virginia result:
Obama: 1,971,820 (51.2%)
Romney: 1,822,522 (47.3%)
Others: 60,147 (1.6%)
So in conclusion it would vote more like Michigan, which isn't completely safe, but it would definitely help secure many democratic victories to come.
So D.C. would be basically Virginia's Detroit politically speaking. It would certainty change things in the General Assembly Dems would probably control both chambers.