The District constituting the seat of Government of the United States shall appoint in such manner as the Congress may direct: A number of electors of President and Vice President equal to the whole number of Senators and Representatives in Congress to which the District would be entitled if it were a State, but in no event more than the least populous State; they shall be in addition to those appointed by the States, but they shall be considered, for the purposes of the election of President and Vice President, to be electors appointed by a State; and they shall meet in the District and perform such duties as provided by the twelfth article of amendment.
The Congress shall have power to enforce this article by appropriate legislation.
Proposed by the Congress in June 1960, it was ratified as an amendment on March 29, 1961. In its ratification, it acknowledged the permanent residents of the District of Columbia should have a voice in electing the President.
What had been created as a monument city to house the government was now a major metropolitan area with a permanent population the size of which the founding fathers could not have imagined. It came to this point with the expansion of the Federal government and the many agencies the executive branch had created, as well as the migration of the blacks from the rural south to the metropolitan areas where better work and pay might be found.
The interesting thing about this city is the pure partisan nature of its population. In all the presidential elections since 1964, its first year of participation, the city has voted exclusively Democrat.
By the way, if you are curious they get three electoral votes -- the same number as Wyoming.
On the bright side, unlike the states where the electors have to travel to DC to do their thing, the district’s three electors can stay home, hop the metro on the appointed day, and probably be home for dinner.