From the moment that President James K. Polk asked Congress for troops to fight Mexico in May 1846, northern Whigs charged that he intended to seize land from Mexico so that slavery could be extended into it. Since the Democrats had just annexed the new slave state of Texas to the Union over the unanimous opposition of Whigs, northern Democrats, who well knew the extent of antislavery opposition to that annexation among their northern constituents, feared that this Whig charge could be lethal in the impending 1846 congressional elections if it were not rebutted. Like their southern colleagues, they supported the war and expected to extract some of Mexico's land as a result of it, but they sought to assure the northern public that slavery would not be allowed to expand into any of the acquired territory.

Their opportunity to do so came in the House of Representatives in August 1846 when a freshman Democrat from Pennsylvania named David Wilmot introduced an amendment to an appropriation bill requested by Polk. Famous ever since as the Wilmot Proviso, the amendment stipulated that slavery and involuntary servitude would be barred from all lands acquired from Mexico as a result of the war. At that time and every time it was reintroduced over the next four years, the Wilmot Proviso split both Whigs and Democrats along sectional lines and polarized Northerners and Southerners against each other. It often passed the House, where Northerners who supported it outnumbered the Southerners who opposed it, only to be buried in the Senate, where the two sections had an equal number of seats. By the start of 1850, 14 of 15 northern state legislatures had instructed their states' congressmen to impose the Proviso on any territories organized in the Mexican Cession while increasing numbers of Southerners vowed to secede should Congress ever pass it into law.

Split irreparably over the Proviso itself, both Whigs and Democrats sought a different formula on the territorial/slavery extension issue on which their northern and southern members could reunite. Until the Treaty of Guadelupe-Hidalgo was ratified in March 1848, Whigs rallied behind a demand that no territory whatsoever be taken from Mexico as a result of the war. Democrats endorsed a position known as popular sovereignty which would remove the decision about slavery in the territories from Congress and leave it to the residents who settled in those territories. To preserve unity in the presidential campaign of 1848, moreover, neither Democrats nor Whigs endorsed the Proviso, causing outraged antislavery men in the North to form the new Free Soil party. Its central platform commitment was to congressional prohibition of slavery from all federal territories, a position that Lincoln and the Republican party would adopt in the 1850s.