Admission of new states
By the 19th century the United States was rapidly expanding westward with new states being formed and admitted into the Union. The two coalitions of Free and Slave states jockeyed to maintain power in the Senate with newly formed states joining either coalition. This balance of power was broken by 1958, favoring the Free State Coalition.
The general policy established was that there would be an equal number of free and slave states in the Senate as more states joined the Union. The 1820 Missouri Compromise was an attempt of maintaining such a balance, admitting Maine as a free state and Missouri as a slave state. The compromise notably provided the condition of the 36° 30' parallel division, states north of the parallel being free and states south being slave states. However, other controversial developments that sought to alter this compromise emerged. The 1846 Wilmot Proviso was a failed proposal that sought to ban slavery across all territory acquired from the Mexican American War under the pretence that Congress could legally restrict slavery but could not establish it. The 1854 Kansas-Nebraska Act asserted the doctrine of “popular sovereignty,” where settlers of a territory could elect whether their state would allow the institution of slavery or not. This notably led to waves of violence and struggles for electoral control in “Bleeding Kansas” that lasted from 1854 to 1861, when Kansas was finally admitted as a free state. With the admittance of California (1850) and Minnesota (1858) into the Union as free states, the balance was lost with a total of 15 slave states to 17 free states, deeply angering slave states who saw the North as unjustly imposing power over the South.
While these debates were centered around the balance of power between two coalitions, the creation of these two coalition was based on whether the state supported the institution of slavery or not. At the heart of these territorial disputes were the debates of slavery.
[P1] Northern states sought to impose their power over Southern states through gaining a majority in the Senate. [P2] States alone have the power to democratically elect whether to establish specific institutions or not.
Rejecting the premises
[Rejecting P2] Slavery was still the underlying cause of these issues, not states' rights themselves.