Wilmot Proviso Defeated: Congress Refuses to Limit Slavery in Mexican Cession

Congressman David Wilmot introduced a proviso to ban slavery from all territory acquired from Mexico in the Mexican-American War. The House passed it twice; the Senate killed it. The Wilmot Proviso's failure — and the Senate's consistent blocking of any restriction on slavery's expansion — demonstrated the extent of slaveholder political power. The debate fractured both parties along sectional lines and foreshadowed the collapse of the Second Party System. The Mexican Cession ultimately added California, New Mexico, Arizona, Nevada, Utah, and Colorado — all contested territory for slavery.