At the end of the Mexican War, many new lands west of Texas were yielded to the United States, and the debate over the westward expansion of slavery was rekindled. Southern politicians and slave owners demanded that slavery be allowed in the West because they feared that a closed door would spell doom for their economy and way of life. Whig Northerners, however, believed that slavery should be banned from the new territories. Pennsylvanian congressman David Wilmot proposed such a ban in 1846, even before the conclusion of the war. Southerners were outraged over this Wilmot Proviso and blocked it before it could reach the Senate.
The Wilmot Proviso justified Southerners’ fears that the North had designs against slavery. They worried that if politicians in the North prevented slavery from expanding westward, then it was only a matter of time before they began attacking it in the South as well. As a result, Southerners in both parties flatly rejected the proviso. Such bipartisan support was unprecedented and demonstrated just how serious the South really felt about the issue.
The large land concessions made to the U.S. in the 1848 Treaty of Guadalupe Hidalgo only exacerbated tensions. Debates in Congress grew so heated that fistfights even broke out between Northerners and Southerners on the floor of the House of Representatives. In fact, sectional division became so pronounced that many historians label the Mexican War and the Wilmot Proviso the first battles of the Civil War.
Even though the Wilmot Proviso failed, the expansion of slavery remained the most pressing issue in the election of 1848. The Whigs nominated Mexican War hero General Zachary Taylor, a popular but politically inexperienced candidate who said nothing about the issue in hopes of avoiding controversy.
The Democrats, meanwhile, nominated Lewis Cass. Also hoping to sidestep the issue of slavery, Cass proposed allowing the citizens of each western territory to decide for themselves whether or not to be free or slave. Cass hoped that a platform based on such popular sovereignty would win him votes in both the North and South.
The election of 1848 also marked the birth of the Free-Soil Party, a hodgepodge collection of Northern abolitionists, former Liberty Party voters, and disgruntled Democrats and Whigs. The Free-Soilers nominated former president Martin Van Buren, who hoped to split the Democrats. He succeeded and diverted enough votes from Cass to throw the election in Taylor’s favor. (Taylor, however, died after only sixteen months in office and was replaced by Millard Fillmore.)