The existing 50 UNITED States of America have 100 elected Senators representing their sovereign, federated interests in the Union headquartered in Washington, the District of Columbia.
The admission of any new additional state, or states into the Union is subject to the consent of a minimum of at least 38 states represented by the unanimous vote of their 2 respective Senators. Since the U.S. Constitution governing the UNITED several states requires that 3/4 of the existing states ratify ANY amendment to the Constitution - admission to that very Union necessarily requires the same 3/4 consenting approval.
The former President John Quincy Adams , and subsequent member of the House of Representatives so argued in 1845 against the admission of Texas into the Union by a mere Joint Resolution of Congress.
Otherwise a cabal of like-minded states could achieve political dominance by conspiring to admit new states sympathetic to their agenda - thus threatening the very fragile, and tenuous alliance enjoyed by the Union.
Washington, the District of Columbia can never be admitted as a state because it exists by definition as the very seat of government for the already existing 50 United States of America.
Accordingly, this is why many citizens of the Republic of Texas still very seriously entertain the notion of withdrawing their participation in that Union at some point in time.