Victory over Mexico added more than i million square miles to the United States—an area larger than the Louisiana Purchase. But the acquisition of this vast territory raised the fatal issue that would disrupt the political system and plunge the nation into civil war—whether slavery should be allowed to expand into the West. Events soon confirmed Ralph Waldo Emerson’s prediction that if the United States gobbled up part of Mexico, “it will be as the man who swallows arsenic.... Mexico will poison us.”
Before 1846, the status of slavery in all parts of the United States had been settled, either by state law or by the Missouri Compromise, which determined slavery’s status in the Louisiana Purchase. The acquisition of new land reopened the question of slavery’s expansion. The divisive potential of this issue became clear in 1846, when Congressman David Wilmot of Pennsylvania proposed a resolution prohibiting slavery from all territory acquired from Mexico. Party lines crumbled as every northerner, Democrat and Whig alike, supported what came to be known as the Wilmot Proviso, while nearly all southerners opposed it. The measure passed the House, where the more populous North possessed a majority, but failed in the Senate, with its even balance of free and slave states. The Proviso, said one newspaper, “as if by magic, brought to a head the great question that is about to divide the American people.”
In 1848, opponents of slavery’s expansion organized the Free Soil Party and nominated Martin Van Buren for president and Charles Francis Adams, the son of John Quincy Adams, as his running mate. Democrats nominated Lewis Cass of Michigan, who proposed that the decision on whether to allow slavery should be left to settlers in the new territories (an idea later given the name “popular sovereignty”). Van Buren was motivated in part by revenge against the South for jettisoning him in 1844. But his campaign struck a chord among northerners opposed to the expansion of slavery, and he polled some 300,000 votes, 14 percent of the northern total. Victory in 1848 went to the Whig candidate, Zachary Taylor, a hero of the Mexican War and a Louisiana sugar planter. But the fact that a former president and the son of another abandoned their parties to run on a Free Soil platform showed that antislavery sentiment had spread far beyond abolitionist ranks. “Antislavery,” commented Senator William H. Seward of New York, “is at length a respectable element in politics.”