"But now how do you propose this very idea before Congress?"
I guess that, in order for it to pass, it would help if it was introduced in the House by DC Delegate Holmes-Norton and co-sponsored by a bipartisan group of DC Metro Area Senators and Representatives.
Among MD Congressmen, I would assume that black Democrat Albert Wynn of PG County and white Democrat Chris Van Hollen of Montco would be in favor, since they would presumably have a good chance of being elected Governor or Senator in a large State of New Columbia, plus their districts would become more of a focal point for the new state than they currently are in Baltimore-centered Maryland. Veteran white Democrat Steny Hoyer of PG County and Southern MD, though, would probably oppose the plan, since he would suddenly find himself without a district; his portion of PG County would surely be in a black-majority district in New Columbia, and the other areas Hoyer represents would end up in a GOP-leaning district centered either in the Eastern Shore or in Annapolis, or (more likely) split among two districts. Maryland's Democrat Senators (currently two, but hopefully only one commencing in 2007) would probably oppose the measure, since it would make it very difficult for them to win reelection; if Lt. Gov. Steele is elected to the Senate, he would be best served to move a bit north or east and support the change (which would make his reelection much easier, as well as greatly improving the chances of all MD Republicans).
As far as VA Representatives are concerned, I would think that Democrat Jim Moran would support the bill, since it would make his district an integral part of the new state and would guarantee him a district tailor-made for him. Republican Frank Wolf would perhaps support it as well, since his district (which is becoming less Republican by the day) would become centered in Loudon County (more Republican than Fairfax) and other parts of NW VA and would probably be safer. Less likely to support the move would be Republican Tom Davis, who represents a swing district in Fairfax and Prince William and who may end up either running against an incumbent in a GOP primary in VA or running in a district of indeterminate composition in New Columbia; more importantly, his dreams of one day becoming a VA Senator would be gone, since he couldn't win a GOP statewide primary if Fairfax and the RINO and Democrat suburbs were carved out of the state. The VA Senators would likely be split, with George Allen favoring the move (which would make him unbeatable for the Senate and would give him a good chance of carrying MD in a presidential run) and John Warner being opposed (without Northern Virginia, he would be vulnerable to a conservative challenger in a GOP primary, and besides it would return Virginia to its Southern roots and away from the "Northeasternism" that Warner espouses.
So it might be possible to get a DC-area bipartisan coalition together for a State of New Columbia, and it would then be doable to convince black Democrats and white Democrats in districts with significant black populations to join Republicans in getting 2/3 of each House to approve a constitutional amendment eliminating DC's 3 electoral votes and establishing the State of New Columbia. Getting approval from the MD and VA legislatures, though, may be more difficult.
BTW, one way to increase both black supporters of a big State of New Columbia and Republican supporters of the move would be if not all of Montgomery County, MD and Fairfax County, VA are appended to New Columbia. If white and Republican parts of West and NW Montco and West and South Fairfax are kept within MD and VA, respectively, it would make the new MD and VA even more Republican, while increasing the black population of the new state so as to make it likelier that a black Democrat is elected Governor or Senator.