Ordinarily, admitting a new state just requires a majority vote of both houses of Congress. The only exception is if a state is formed out of part of an existing state, in which case that state's legislature must consent. (U.S. Const., Art. IV).
D.C., however, is a whole different kettle of fish, because its status is already governed by the 23rd Amendment, so this would require a constitutional amendment, which isn't going to happen.
Twenty-Third Amendment - U.S. Constitution
Section 1. 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 twelth article of amendment.
Section 2. The Congress shall have power to enforce this article by appropriate legislation.
23rd Amendment Annotations
Enfranchisement of Residents of District of Columbia
‘’The purpose of this. . . constitutional amendment is to provide the citizens of the District of Columbia with appropriate rights of voting in national elections for President and Vice President of the United States. It would permit District citizens to elect Presidential electors who would be in addition to the electors from the States and who would participate in electing the President and Vice President.
‘’The District of Columbia, with more than 800,000 people, has a greater number of persons than the population of each of 13 of our States. District citizens have all the obligations of citizenship, including the payment of Federal taxes, of local taxes, and service in our Armed Forces. They have fought and died in every U.S. war since the District was founded. Yet, they cannot now vote in national elections because the Constitution has restricted that privi lege to citizens who reside in States. The resultant constitutional anomaly of imposing all the obligations of citizenship without the most fundamental of its privileges will be removed by the proposed constitutional amendment. . .
‘’[This] . . . amendment would change the Constitution only to the minimum extent necessary to give the District appropriate participation in national elections. It would not make the District of Columbia a State. It would not give the District of Columbia any other attributes of a State or change the constitutional powers of the Congress to legislate with respect to the District of Columbia and to prescribe its form of government. . . . It would, however, perpetuate recognition of the unique status of the District as the seat of Federal Government under the exclusive legislative control of Congress.’’ 1
Well that’s a relief. Last thing we need is for another state to give 4 more electoral votes to the Democrats.