[Brilliant]

I don’t think it would require approval of the states involved. You can’t make DC a state without a constitutional amendment anyway. So the provision that requires state approval would be overrriden as well when you amend the Constitution.

[scrabblehack]

I.8.xvii:
To exercise exclusive Legislation in all Cases whatsoever, over such District (not exceeding ten Miles square) as may, by Cession of particular States, and the Acceptance of Congress, become the Seat of the Government of the United States,

IV.3.i:
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.

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The proposal to reduce the size of the seat of government has been floated before, although it’s usually been floated as ceding the residential areas of D.C. back to Maryland. (Alexandria was ceded back to Virginia in 1846).