https://constitution.congress.gov/brows ... section-8/
I believe they might be able to make MOST of DC a state, but not the entirety of it. I see nothing in the Constitution regarding a vote by the other states to admit a new state. In fact, Article IV, Section 3, Clause 1 states the following;
Therefore, it appears it is solely up to congress as to whether or not to admit a new state.New States may be admitted by the Congress into this Union; but no new State 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.