With the Mexican War, the extension of slavery into the territories became a national issue, and several solutions to the problem were suggested. Shortly after the fighting began, Democrat David Wilmot of Pennsylvania introduced an amendment to an appropriation bill in the House of Representatives calling for the prohibition of slavery in any territory to be acquired from Mexico. Although the Wilmot Proviso
never became law, John C. Calhoun responded to it with a series of resolutions, maintaining that any attempt to ban slavery was unconstitutional: slaves were property, and if a person wanted to take his property to another part of the country, no law could prevent him from doing so. Furthermore, the Fifth Amendment prevented Congress from depriving anyone of their property without due process. On middle ground between these two extreme positions was a proposal for “ squatter sovereignty
” (later known as “ popular sovereignty
”), championed by Lewis Cass of Michigan. Popular sovereignty, if accepted, would let the settlers themselves decide whether slavery would be allowed in their territory.
The election of 1848. With his foreign policy objectives achieved, Polk decided not to run for a second term. Zachary Taylor was the nominee of the Whigs. Although himself a slaveowner, he had not taken a public stand on slavery or any other major issue of the day and, in fact, had never voted in a national election. The Whigs had no party platform and ran the campaign solely on Taylor's war record. The Democrats chose Lewis Cass, but their platform called on Congress not to interfere with slavery and did not mention popular sovereignty. The wild card in the election was the Free‐Soil party, a coalition of three groups: dissident Democrats who supported the Wilmot Proviso, members of the abolitionist Liberty party, and anti‐slavery Whigs from New England.
The major parties ran a distinctly sectional campaign. In the North, the Whigs claimed that Taylor would back the Wilmot Proviso if Congress approved it, while they reminded southern voters that their candidate was a son of the South. The Democrats assured both parts of the country that the territories would decide the slavery question on their own without Congress, leaving northerners to believe that the West would be free and southerners confident that slaves would be allowed. The results of the election showed the effects of the campaign. Taylor won the presidency with 163 electoral votes (eight slave and seven free states) to Cass's 127 (seven slave and eight free states); the Free‐Soil party did not win any states but did split the vote in New York to Taylor's favor and the Ohio vote in Cass's.
The California gold rush. In January 1848, gold was discovered in California. The news spread around the world and was confirmed by President Polk in his annual message to Congress in December. Tens of thousands of people, mostly white Americans, flooded into California, looking to make their fortune in the gold fields; a polyglot mix of free African Americans, Mexicans, Pacific Islanders, and Europeans rushed in as well. With the influx of the forty‐niners, who were chiefly young men without families, the population of California reached one hundred thousand by the end of 1849 and continued to grow. Easy‐to‐locate gold deposits were soon played out, and by 1852, many miners found themselves wage earners for highly mechanized and well‐financed mining operations. Others gave up prospecting soon after they arrived in California, realizing that more money could be made in providing food, lodging, and other services to the new arrivals.
The economic and social impact of the gold rush was less important at the time than California's political future. A state constitution that prohibited slavery was adopted in the fall of 1849, and in December, President Taylor recommended that California be admitted into the Union. Admission was a volatile issue because the numbers of slave and free states were balanced at fifteen each. Oregon had been organized as a free territory in 1848 on the basis of its provisional constitution and the fact that it lay north of the line established by the Missouri Compromise of 1820. Extending that line—36°30′ north latitude—to the Pacific would have cut California in two. It fell to Congress, which had scrupulously tried to avoid the slavery question for almost three decades, to decide slavery's fate in California and the rest of the Mexican Cession.