Equal footing

The equal footing doctrine, also known equality of the states, is the principle in United States constitutional law that all states admitted to the Union under the Constitution since 1789 enter on equal footing with the 13 states already in the Union at that time. The Constitution grants to Congress the power to admit new states in Article IV, Section 3, Clause 1, which states:

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.

Beginning with the admission of Tennessee in 1796, Congress has included in each state's act of admission a clause providing that it enters the Union "on an equal footing with the original States in all respects whatever". Before that, when Vermont was admitted in 1791, the act said Vermont was to be "a new and entire member" of the United States.

At the 1787 Constitutional Convention, a proposal to include the phrase, "new States shall be admitted on the same terms with the original States", was defeated. It was feared that the political power of future new western states would eventually overwhelm that of the established eastern states. Once the new Constitution went into effect, however, Congress admitted Vermont and Kentucky on equal terms and thereafter formalized the condition in its acts of admission for subsequent states, declaring that the new state enters "on an equal footing with the original States in all respects whatever." Thus the Congress, utilizing the discretion allowed by the framers, adopted a policy of equal status for all newly admitted states. With the growth of states' rights advocacy during the antebellum period, the Supreme Court asserted, in Lessee of Pollard v. Hagan (1845), that the Constitution mandated admission of new states on the basis of equality".

...
Wikipedia