Thomas Jefferson, who had drafted the clause of the Northwest Ordinance of 1787 prohibiting slavery north of the Ohio River, strenuously opposed efforts to keep the institution out of Missouri. He saw the entire controversy as an attempt by Federalists to revive their party by setting northern and southern Republicans against each other. Jefferson was correct that political power, not moral scruples, motivated most northern congressmen. But Republicans, not the few remaining Federalists, provided the bulk of the votes against slavery in Missouri. By 1820, New York had surpassed Virginia in population, and New York Republicans were among the leading advocates of emancipation in Missouri. Twenty-eight years of Virginia presidents, interrupted only by the single term of John Adams of Massachusetts, had persuaded many northerners that the South exercised undue influence in Washington. More slave states meant more southern congressmen and electoral votes.
The Missouri controversy raised for the first time what would prove to be a fatal issue—the westward expansion of slavery. The sectional division it revealed aroused widespread feelings of dismay. “This momentous question,” wrote Jefferson, “like a fire bell in the night, awakened and filled me with terror. I considered it at once as the knell of the union.” John Quincy Adams wrote of the debate in his diary:
[It] disclosed a secret: it revealed the basis for a new organization of parties.... Here was a new party really formed... terrible to the whole Union, but portentously terrible to the South—threatening in its progress the emancipation of all their slaves, threatening in its immediate effect that southern domination which has swayed the Union for the last twenty years.
The “dissolution of the Union” over the issue of slavery, Adams mused, disastrous as that might be, would result in civil war and the “extirpation of slavery from this whole continent.” It would take more than forty years for Adams’s prediction to be fulfilled. For the moment, the slavery issue faded once again from national debate.