Answer: There are several aspects of American history that explain the relative balance between states that abolished slavery and those that retained it during much of the time between the ratification of the Constitution and the Civil War. Various compromises allowed new states to enter the Union after the original thirteen in order to keep the balance. The maintenance of the balance was tied to the Constitution and how the document organized Congress. While a balance was maintained for much of the antebellum period, the various compromises conceded much to the South. The delicate balance eventually crumbled and led to the Civil War.
In order to understand the balance of power between the slave and free states, the original thirteen states are the proper starting point. States began moving toward abolition of slavery during the American Revolution. For instance, by the mid-1780s five of the original thirteen states (New Hampshire, Massachusetts, Pennsylvania, Connecticut, and Rhode Island) had abolished slavery in their state constitutions. In addition, Vermont, which became the fourteenth state to join the Union, had abolished slavery in its constitution in 1777. However, not all Northern states initially wanted abolition. New York and New Jersey had large slave populations in the late eighteenth century and were reluctant to abolish slavery, which was evident by the failure of several gradual emancipation measures. After the initial admission of the original thirteen colonies to the Union by Constitutional ratification, the balance between slave states and free states was roughly maintained by one slave state and one free state being admitted to the Union within a year of each other. The only breaks in this pattern were the admissions of Tennessee, Ohio, and Louisiana, which were separated by years between their admissions. The pattern would return with the admission of Indiana and Mississippi in 1816 and 1817 respectively.
The continuation of the balance between slave and free states was important given the nature of the Congress and the balance of power between the competing sides in the issue. Given the proportional representation of the House of Representatives and the arguably overrepresentation of Southern states via the Three-Fifths Compromise, the balance between the states was very important, as each side did not want the other to have a majority of power in Congress. If there were even one more slave state or free state, the balance of power would shift in the Senate, and would likely shift in the House as well. Compromises were needed to equalize the power between proslavery and antislavery interests in the government to keep the Union together.
