Adding DC to Maryland might actually cuts both ways. Adding DC to Maryland would most likely increase Maryland's representation in Congress by one and take a seat away from another state (possibly a GOP seat, a plus for Democrats). However, in presidential elections, DC right now gets 3 electoral votes. Adding it Maryland would probably boost Maryland electoral votes by one, but the Democrats would lose the 3 electoral votes it currently gets every election from DC. (a net +2 or +3 electoral votes for the GOP).

Does anyone know what it would take for DC to become part of Maryland? A simple majority in Congress or 2/3 of Congress? Do the other 50 states have to approve it as well?

Article IV, Section 3 has the process:

Section 3. New states may be admitted by the Congress into this union; but no new states shall be formed or erected within the jurisdiction of any other state; nor any state be formed by the junction of two or more states, or parts of states, without the consent of the legislatures of the states concerned as well as of the Congress. The Congress shall have power to dispose of and make all needful rules and regulations respecting the territory or other property belonging to the United States; and nothing in this Constitution shall be so construed as to prejudice any claims of the United States, or of any particular state.

So, a simple majority vote would do it. Congress is composed of representatives from the states, so, yes, indirectly.

you forgot the most politically active sub-million (600K) state with the most noticeable politicians... Vermont.

And the population has only really grown since the 1960s...

There might be an effective remedy to the squatting problem (redesignating the District as an unoccupied atoll, preferably one that's highly radioactive) but I'm not sure how you deal with the electoral votes under a retrocession situation (since they'd still count toward the 270-vote threshold, but nobody could win them).

The other problem is: what happens if the federal government cedes its claim to D.C. but Maryland (quite sensibly, I might add) refuses to take it back? Does Virginia get dibs?

Yes, you make a good point on the Electoral Votes. Presumably, we'd have to repeal that amendment in exchange for them getting representation in Congress.

I would think Maryland would be glad to have the District, since it would mean another Representative and more federal revenue, but maybe not. I guess Virginia would be the most logical recepient at that point, even though they have no claim to that land.

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