US Constitution Series, Article VI, Section 3
Article IV. STATES' RELATIONSThis has interesting possibilities. In the surge of adding states in the 19th century, the battles were over the balance of political power, and before the civil war, over slave versus non-slave states. Consolidating, say, Rhode Island, Connecticut and New York without their consent would substantially increase the red state influence in the Senate by eliminating four blue state senators. It cannot be done without the consent of the state legislatures involved. Not going to happen.
§ 3. Admission of New States to Union; Property of United 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.
The Congress shall have Power to dispose of and make all needful Rules and Regulations respecting the Territory or other Property belonging to the United States; and nothing in this Constitution shall be so construed as to Prejudice any Claims of the United States, or of any particular State.