The Free Soil position had a popular appeal in the North that far exceeded the abolitionists’ demand for immediate emancipation and equal rights for blacks. While Congress possessed no constitutional power to abolish slavery within a state, well-known precedents existed for keeping territories (areas that had not yet entered the Union as states) free from slavery. Congress had done this in 1787 in the Northwest Ordinance and again in the Missouri Compromise of 1820-1821. Many northerners had long resented what they considered southern domination of the federal government. The idea of preventing the creation of new slave states appealed to those who favored policies, such as the protective tariff and government aid to internal improvements, that the majority of southern political leaders opposed.
For thousands of northerners, moreover, the ability to move to the new western territories held out the promise of economic betterment. The depression of the early 1840s had reinforced the traditional equation of land ownership with economic freedom. The labor movement promoted access to western land as a way of combating unemployment and low wages in the East. “Freedom of the soil,” declared George Henry Evans, the editor of a pro-labor newspaper, offered the only alternative to permanent economic dependence for American workers.
Such views merged easily with opposition to the expansion of slavery. If slave plantations were to occupy the fertile lands of the West, northern migration would be effectively blocked. The term “free soil” had a double meaning. The Free Soil platform of 1848 called both for barring slavery from western territories and for the federal government to provide free homesteads to settlers in the new territories. Unlike abolitionism, the “free soil” idea also appealed to the racism so widespread in northern society. Wilmot himself insisted that his controversial Proviso was motivated not by “morbid sympathy for the slaves” but to advance “the cause and rights of the free white man,” in part by preventing him from having to compete with “black labor.”
To white southerners, the idea of barring slavery from territory acquired from Mexico seemed a violation of their equal rights as members of the Union. Southerners had fought and died to win these territories; surely they had a right to share in the fruits of victory. To single out slavery as the one form of property barred from the West would be an affront to the South and its distinctive way of life. A majority of slaves in 1848 hved in states that had not even existed when the Constitution was adopted. Many older plantation areas already suffered from soil exhaustion. Just as northerners believed westward expansion essential to their economic wellbeing, southern leaders became convinced that slavery must expand or die. Moreover, the admission of new free states would overturn the delicate political balance between the sections and make the South a permanent minority. Southern interests would not be secure in a Union dominated by non-slaveholding states.