The Constitution says that states are not to be partitioned, but Kentucky and West Virginia have split off from Virginia, as well as Maine from Massachusetts. I also know that currently as well as in the past, there have been lots of campaigns for splitting off parts of a state (rural California, western Washington, southern Ohio, etc.). What is the history behind this seeming contradiction of the Constitution saying one thing but no one seeming to acknowledge it? Does it/has it actually prevented states from partitioning, and if so, why were the three successful cases exceptions? Thanks in advance.
"The Constitution says that states are not to be partitioned"?! Quite the contrary. Article 4, section 3:
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.
For the three cases:
Virginia passed an enabling act to allow Kentucky to be its own state. Well, actually, four, though because reasons only the last one actually got the agreement of Kentucky and the new Congress. "An act concerning the erection of the district of Kentucky into an independent state", 18 December 1789. Congress accepted it 4 February 1791.
Maine had a process to split from Massachusetts, but that would have altered the balance between slave and free states, so it wasn't admitted until the Missouri Compromise also admitted Missouri as a slave state in 1820. The admission of Maine happened starting with an act of the Massachusetts General Court, a plebicite in Maine, a Maine Constitutional Convention, and the acceptance of its separation again by Massachusetts.
Virginia and West Virginia ... that was a lot of sharp practice. After Virginia passed an ordinance of secession, delegates from the western parts of Virginia met at Wheeling, in two successive conventions. The second one declared that the secession was against the constitution of Virginia (!) because any change had to be approved by the people -- and though the convention had been elected by the people and the ordinance approved by plebicite, the calling of the convention had been done by the legislature, not the people, rendering everything following invalid (!!). It declared all acts of the seceding legislature to be invalid and the offices of those upholding secession voided, and established the "Restored Government of Virginia" with remaining Virginia legislators and an interim governor. This Restored Government got the recognition of Congress and the seating of its US Senate candidates. They proceeded to approve a "State of Kanawha", constituting most of the counties that the Restored State controlled or claimed to control, and after more irregularity of votes for conventions and constitutions, West Virginia was established and admitted in 1863. The Restored Government moved to Alexandria, VA, though it had authority in only few counties, had no Representatives in Congress and two Senators (then one when one died) in 1863-5, then no members at all from 1865. As the price of readmission of Virginia, Congress required that Virginia's 1869 constitution recognize as valid the split of West Virginia.