In retrospect, the more that he pondered its dynamics, the more certain Thomas Jefferson grew that the Missouri Crisis had been orchestrated for political purposes by a group of sly Federalist operatives. "The Missouri question is a mere party trick," he wrote to Charles Pinckney, several months after Congress formulated the well known 36 °30' line. "The leaders of federalism, defeated in their schemes of obtaining power by rallying partisans to the principle of monarchism ... are taking advantage of the virtuous feelings of the people to effect a division of parties by a geographical line." Former President James Madison agreed that the proponents of slavery restriction shared "an object very different from the welfare of the slaves," while his successor and Virginia neighbor, President James Monroe, was certain that the sole intent of the antislavery forces was "undoubtedly to acquire power."As time went on, this position looked more and more like a cynical defense of slavery. Which it surely always was to some degree, depending on the person.
But politics is a tribal affair, as we are constantly reminded. Partisan loyalties grow over time and become connected to a variety of social, cultural, class and political interests and issues. And there is also a strong temptation to regard the other side as acting in bad faith in their use of issues. Which, as we see regularly, is often actually the case.
The antislavery issue became associated with Northern Democrats, the remnants of the Federalist Party (notably the archetypical conservative Daniel Webster), the Whigs and eventually the Republicans. But the antislavery advocates were also often associated with a tendency to favor government by wealthy industrial and banking elites, support for the Bank of the United States, and opposition to the expansion of the right to vote. The sectional alignments were primarily though not exclusively based on slavery, but sectionalism also functioned as a distinct source of political alignments.
We shouldn't lose sight on the main point, that slavery was wrong and incompatible with democracy and democratic freedoms. As we've also seen, the ways in which the slave system in practice imposed more and more obvious restrictions on democracy for whites in both slave and free states. And that meant that white voters had a rational selfish reason for hating slavery. That meant that antislavery sentiment could be and often was associated with hatred of black people, as well. Or, in another way of putting it, white voters and politicians could be "sincerely" and practically opposed to slavery and/or its expansion, while not being "sincere" in expressing concern for the well-being of the black slaves themselves.
By December 1820 [the year the Missouri Compromise was concluded] Jefferson could assure his old friend, Marquis de Lafayette, that the debate had not been "a moral question, but one merely of power." Eight months later he detected no more lasting harm done than that the episode had "given resurrection to the Hartford Convention men." Some three years after the crisis had passed, the sage of Monticello remained indignant that "the people of the North [had been] blindfolded into the snare [and had] followed their leaders for awhile with a zeal truly moral and laudable, until they became sensible" that the Missouri question had been "got up" under a "false front."We don't have to take such claims as those by Jefferson just quoted at face value. He was a retired politician, but very much a politician, diplomat and polemicist nevertheless. However, understanding the role that this claim played is important to understanding the evolution of the slavery debate at that time.