Great in theory, but about as far from political reality as you can possibly get. What makes you think the Dems in Congress would ever in a million years agree to a plan that would lead to a net loss of Dem electoral votes and Senate seats? Additionally, any change in the boundaries of a state must be approved by the legislature. Assuming Virginia might be persuaded to go along (extremely doubtful) there's no way in hell that Maryland's Democratic legislature would. Finally, the Washingtonians wouldn't want it, because they'd be in a state of 5 million or whatever, where suburbanites would hold all the power.
It's just dicking around with maps and data for fun. No one expects it would really happen.
Congressional Democrats would probably bet on the smaller MD voting Democrat, pointing to 1992, 1996 and 2000, and that would give Democrats a 16-11 EV advantage as compared to the current 13-13 tie. They would also assume that MD would elect 2 Dem Senators, as would New Columbia, giving them a 4-2 advantage in Senators (as opposed to the usual 2-2 deadlock under current boundaries, and no worse than the 3-1 advantage they currently have after their freak Senate win in VA).
DC residents would be far better off under New Columbia than under the alternatives (assuming that DC statehood is an impossibility, which it is), since they don't get congressional representation or a decent tax base by staying a federal district and they would be completely ignored in the political process if DC was returned to Maryland (remember, MD politicians consider the DC suburbs foreign territory, so you can imagine how they would treat DC). In a state of New Columbia, DC would be the central focus of political and economic attention, since most residents of the new state would work directly or indirectly for the federal government (many of them in DC itself). I think that, after 200 years of disenfranchisement, DC voters would jump at the chance of forming part of a state of New Columbia.
You're correct that the hardest part of getting the proposal approved is the apprehension from the VA and MD state legislatures. The DC suburbs form an important part of the tax base of each state, although culturally its residents are quite different from those of the rest of the state. But I think that Northern Virginians, including their state legislators, would rather form part of a DC-centered state than one whose political power lies mostly in the Richmond area, Hampton Roads, Souhtside and Appalachia, and that state legislators from the rest of the state would be willing to get those areas out of their hair (especially since they'd keep taxpayers in suburban Loudon and Prince William Counties).
Maryland is trickiest of all, of course, since its heavily Dem legislature may be unwilling to turn the state into a swing state. The trick would be for DC suburbanites to take the initiative to form part of New Columbia, and legislators tired of that whole "superconnector" debate and other suburban pet issues may be willing to let them go. A "Keep Maryland for the Marylanders" campaign complete with Chesapeake Bay crabs and Pimlico Race Course, pointing out the advantages of a smaller, less "federal," Maryland, would be helpful as well.
I think the odds of New Columbia being acceptable to all sides are not as low as they are for the return of DC to MD would be.