Wilmot Proviso
Anti-slavery elements fought for the exclusion of slavery from any territory absorbed by the U.S. In 1847, the House of Representatives passed the Wilmot Proviso, stipulating that none of the territory acquired should be open to slavery. If successful, the Wilmot Proviso would have effectively cancelled out the 1820 Missouri Compromise, since it would have prohibited slavery in an area below the parallel 36°30′ north. The Senate avoided the issue, and a late attempt to add it to the Treaty of Guadalupe Hidalgo was defeated because Southern Senators had the votes to prevent its addition. The House of Representatives is apportioned by population, and the North's was growing, allowing it to win the majority of the House in the 1846 elections; but the Senate representation is two per state and Southerners had enough votes to block the addition.