There had been comparatively little anti-slavery agitation thus far, being confined to attacks upon the slave trade and an occasional petition from the Friends; yet the sentiment that slavery was an economic evil was firmly established in the overstocked border slave States, and that it was both an economic and moral evil was believed by a growing number in the Northern States. The "Lower South," or Gulf States, were thus left as the guardians of a system which the increasing cultivation of cotton in that region made unusually profitable and, as they thought, indispensable. Missouri lay far to the north of them, but the maintenance of political power in the Union was essential to their future if they read aright the growing hostile sentiment of the North. Immediate or gradual emancipation had been provided by every old State in the North, and slavery had been prohibited by the constitutions of the new Northern States. Feeling the approval of a good conscience, it was probable that the North would eventually demand a kindred movement in the South. There is no reformer likely to be so intolerant as the one who has left off what he considers a bad habit.
The slavery system had been so thoroughly rooted in colonial times and so freely recognised and protected in the Constitution, that few as yet contemplated interfering with it in any State where it already existed. Home rule and individual rights were too sacred for that. Majority rule had not yet made sufficient headway against individualism. But the Union had a kind of prenatal control which it could exercise over States created from Territories. Here was an opportunity to exercise it. An early attempt was made in Congress on the part of those hostile to the extension of slavery to make Missouri a free State by prohibiting "the further introduction of slavery or involuntary servitude." It was met by a counter amendment from the pro-slavery people jointly admitting Maine and Missouri with no such restrictions. This would evidently throw Missouri open to slavery.
The ensuing debates in Congress, covering parts of two sessions, opened all the sectional dissensions, showed how weak were the ties of unionism thus far developed, cut sharp lines across political parties, and shifted the old East and West sectional danger to North and South. The phrase "Mason and Dixon line" was used to express the sectional demarcation, transformed to that use, it is said, by John Randolph. Recrimination and abuse were common. Northern speakers drew insulting comparisons between the population, wealth, and prosperity of the free and slave States. They attributed the difference to the blight of slavery. Southern speakers explained that slavery was a thing of which a non-resident could not judge properly; that what appeared to an outsider as a lack of prosperity was the enjoyment of life by a people not devoted to the sordid aspects of existence; that slavery was a matter for home rule and did not concern the other half of the Union. The Northern contingent replied that slavery was a menace to free labour and that their devotion to all parts of the Union, as well as their right of self-preservation, warranted their interference. Then the Southern speakers taunted them with Shays's Rebellion, the whiskey insurrection, and the Hartford Convention, as proofs of their devotion to the Union. The people of New York were reproached with wishing to deprive Southern people of their slave property, although they themselves still held more than ten thousand slaves and held them under protection of the State laws. One Southern speaker came very near the truth when he predicted that the census for 1820 would show fully twenty thousand slaves still held in bondage in the Northern States. A long discussion arose over the number of troops each section had furnished to the Revolutionary War and upon the number of distinguished men bred in each section. The Bible was quoted freely to attack or defend human bondage. Resolutions of State Legislatures added their weight to either side. Some debaters in Congress deplored the "poisoning of the national affection," seeing in it the revival of the sectional envy and dislike dormant for the past thirty years. Other hot-blooded speakers declared that this contest could be ended only by bloodshed.