On March 10, 1848 the Senate ratified the Treaty of Guadalupe Hidalgo by which Mexico ceded to the victorious United States a tract of western land stretching from the Rocky Mountains to the Pacific and from Oregon to the Gulf of California. That summer gold was discovered in California, already the most populous province of this vast region, and tens of thousands more journeyed to the gold fields overwhelming the governing capacities of the U.S. army garrison there.
Eager to furnish civil authorities for California and fearful of prolonged debate about whether the territory would be free or slave, newly elected president Zachary Taylor proposed that Californians skip the territorial phase and petition for statehood, which they promptly did, ratifying an anti-slavery state constitution. In reaction, Southerners hotly complained about lack of Northern reciprocity for their agreeing to the free status of Oregon two years before.
As the 31st Congress convened in November, 1849, Democrats, Whigs, and Free Soil representatives were confronted with other galling problems. Many Northerners adamantly sought to pass the Wilmot Proviso which barred slavery from the rest of the Cession. Southerners rankled about the difficulty of recovering the 60,000 runaway slaves harbored in Northern states and about the decade-long abolitionist petition campaign to end slavery in the District of Columbia. In particular, the opinions on California ranged from making it a territory open to slavery to immediately admitting it to the United States as a free state. As for New Mexico and Utah , proposals ran the gamut from Congress banning slavery in all territories to allowing territorial citizens to decide on their labor system. On the topic of slavery in the District, some demanded suppression of all anti-slavery agitation in the North, while others called for immediate abolition by Congress. Lastly, negotiators disagreed on the extent of the role of the national government in apprehending and returning fugitive slaves.