Wilmot Proviso.

The crisis over the status of slavery in the territory acquired from Mexico grew out of the struggle for partisan advantage. A breach in the Democratic Party opened when James K. Polk defeated former president Martin Van Buren for the presidential nomination in 1844, and the split deepened over the next two years. When Polk guided the United States into a war with Mexico, planning to annex territory extending to the Pacific Ocean, the Van Burenites moved to exact their revenge. Representative David Wilmot of Pennsylvania proposed to add a clause, or proviso, to military appropriations for the war stipulating that slavery would not be permitted in any territory obtained from Mexico. Wilmot's proposal imitated restrictions by which slavery had been barred from the Northwest Territory in 1787 and from the Louisiana Purchase Territory north of the 36° 30' latitude in the Missouri Compromise...