would invite race war, at least in areas with substantial African American populations. The economic impact of losing western slave markets was one thing, but the palpable fear of living among an ever-increasing population of potential rebels—“dammed up in a land of slaves” was how Virginia judge Spencer Roane put it—was something else again. Besides, how would the federal government continue to enforce gradual emancipation in Missouri over the decades after the territory had become a state?

Southern statesmen such as Jefferson, who had publicly deplored slavery for a long time, now found themselves arguing that it would be better were the institution dispersed ever more thinly into newly settled areas rather than concentrated in the older states. “Diffusion” of slaves “over a greater surface,” as Jefferson rationalized it, would “facilitate the accomplishment of their emancipation” by better preparing the local whites to see them freed by spreading the burden of compensating slaveholders. So the extension of slavery, claimed the man who had assured the world that all men were created equal, would actually enhance the long-term prospects of ending slavery. Not surprisingly, Northerners found the argument unconvincing. But even those white Southerners who regretted their problematic institution and hoped to eliminate it would not tolerate Northern participation in planning how to do so.

Northern opposition to the extension of slavery reflected both moral disapproval and jealousy of the slaveholders’ power. The North had a larger population than the South, and consequently more members of the House of Representatives; but there was an equal number of slave and free states, and therefore an even balance between the sections in the Senate. If slavery were on the road to ultimate extinction in Missouri, Northerners hoped the state’s senators might forsake the proslavery bloc.

Voting on the Tallmadge amendment registered sectional polarization. The House of Representatives narrowly approved gradual emancipation 80 Northern votes to 14, the South casting just two votes against 64. But the slave states had greater strength in the Senate; furthermore, three of the four senators from Illinois and Indiana reflected the sentiment of settlers from the South and voted against the amendment. The Senate refused to accept any restriction on slavery. With the two houses deadlocked, the prospects for Missouri statehood looked bleak.

Monroe, Clay, and Senate leaders worked behind the scenes to devise a compromise and break the deadlock. It would center on what is now the state of Maine, which had been part of Massachusetts since colonial times. In June 1819 the Massachusetts legislature consented to separate statehood for what had been its northern “district.” The Senate leadership promptly joined the two admissions into a single bill, which if passed would preserve the concise balance of sections in the Senate.

But the Northern majority in the House remained determined to enforce gradual emancipation, making a further concession necessary. So Sen. Jesse Thomas of Illinois, who had been voting with the proslavery side, proposed that slavery should be prohibited not in Missouri but in all the rest of the Louisiana Purchase

lying north of 36 degrees and 30 minutes of latitude, that is, the southern boundary of Missouri. This would seriously restrict the expansion of slavery, even if not into the land originally in question.

The Thomas proviso passed the House with the support of 95 out of 100 Northern representatives, and even the Southern members supported it, 39 to 37. It is remarkable how many of the Southern congressmen of 1820 were willing to prohibit slavery in what was then the greater part of the territories. Given the Thomas proviso, 18 Northern representatives then either voted for Missouri statehood without restrictions on slavery or else abstained—enough for it to pass with the support of a solid South. In the Senate all the compromise measures were voted on together as a package: the South voted 20 to 2 in favor; the North, 18 to 4 against.

Although most Northern congressmen at the time would have preferred Tallmadge’s policy of gradual emancipation, in practice the Missouri Compromise helped stabilize sectional competition for 34 years. Its repeal in 1854 by the Kansas-Nebraska Act, which revived the specter of slavery’s extension westward, constituted a monumental legislative blunder. The Northern public had counted on the Thomas proviso of the Missouri Compromise to hold the Great Plains safe for family farms and keep out slave-operated plantations. Abraham Lincoln, who had reconciled himself to private law practice, reentered politics to denounce the act. The new Republican Party that he would lead to victory six years later was born in reaction to what its members saw as the overreaching aggression of slavery expansionists. The Republicans reaffirmed the principle that had been maintained by the Missouri Compromise for more than a generation: congressional power to restrict the extension of slavery into new areas.