By Publius Huldah
Under Article IV, Sec. 3, US Constitution, Congress has the power to admit new States into the Union.
For a list of States and when admitted, see this wiki list.
Note that the new States were “colonies” or “territories” – or as with Vermont and Texas, “independent Republics” – before they were admitted into the Union as “States”. That is highly significant.
The District of Columbia has an existing constitutional status as “the Seat of the Government of the United States” – see Article I, Section 8, next to last clause, US Constitution.
In order to change the constitutional status of the District of Columbia from the “seat of government of the United States” to a “State”, Article I, Section 8, next to last clause, must be amended pursuant to Article V, U.S. Constitution.
Any pretended Act of Congress which purports to confer statehood on the District of Columbia would be totally and blatantly unconstitutional.