They will have to amend the Constitution to do it. A DC Statehood Amendment was reported out of Congress to the states for ratification in 1972, but it expired in 1979 with very few ratifications.
The District of Columbia Voting Rights Amendment was proposed by Congress on August 22, 1978. Section 1 of the proposal, its principal part, said:
For purposes of representation in the Congress, election of the President and Vice President, and article V of this Constitution, the District constituting the seat of government of the United States shall be treated as though it were a State.
By the time this proposal died when its seven-year time limit on ratification expired, it had received only 16 ratifications; it needed 38 for adoption. Knowing a solid majority of States were against DC Statehood, or any variant of it, the Democrats have repeatedly tried to get a DC Representative and DC Senators added to Congress via legislation.
I'm glad the House Democrats will hold hearings on this bill. The People, especially the "swing voters", can see how much the Democrats disrespect the Constitution. Fortunately, the Republicans have the Senate and Trump is President.