James Knox Polk was a loyal Jackson Democrat, playing a central role in the Bank War of 1832-3 before becoming the main supporter of the Jackson administration while serving as Speaker of the House of Representatives in Congress. Polk was a slaveholder from Tennessee, where he became governor in 1839; he fashioned himself after Jackson and was an ardent proponent of annexation as a means of expanding the territorial holdings of the United States.
In 1844, the Democratic Party was set to nominate former president Martin Van Buren, himself a savvy politician who had dedicated most of his long career to avoiding a showdown between the North and the South over the issue of slavery. However, when Van Buren came out against a popular push to annex Texas in order to avoid conflict over the slavery issue, he ended up sparking dissension within the ranks of the party. Democratic delegates who supported expansion, including most party members from the South, maneuvered to block Van Buren and instead nominated the pro-expansion Polk as the first so-called "dark-horse" (or underdog) candidate for the presidency.
Polk managed to win over the various feuding factions of Democrats by promising (among other things) that he would only serve for one term and by coming out in support of the Tariff of 1842, despite his anti-protectionist record (Polk later reneged on the tariff issue).
Polk and the Democrats heralded the promise and benefits of territorial expansion above all other matters, hoping that a focus upon the West would drown out the threat of sectional discord between North and South. Democrats referred to the "clear and unquestionable" title that America held over the Oregon Territory (which Britain also claimed to own) and Texas (which had declared itself an independent republic and had been warring with Mexico). In response, Whigs—the main opposition party at the time—taunted Polk for his lack of political stature, particularly in contrast to their own nationally renowned candidate, Senator Henry Clay of Kentucky. Clay tried to appease southern Whigs by saying that he had no personal objection to the annexation of Texas, but that he thought it could destroy the country. In a telling indication of the growing sectional divide, especially among Whigs, Clay vacillated on the annexation issue in a futile attempt to appeal to both the northern and southern contingents of his party. In the end, Polk barely eked out a victory; he lost his own home state of Tennessee but won the national vote by the tiny margin of 1.4 percent, a result made possible in part by the massive turnout of Irish Democrats in New York.
Polk's successful expansionist policies created unexpectedly divisive new political conflicts. In August 1846, an obscure congressman from Pennsylvania, Democrat David Wilmot, tacked a proviso onto an appropriations bill that President Polk had sent to the House. The proviso borrowed language from Thomas Jefferson's Northwest Ordinance, which said that "neither slavery nor involuntary servitude shall ever exist in any part of said territory, except for crime, whereof the party shall first be duly convicted." Wilmot sought to apply this provision to any new territory acquired through the Mexican-American War, thus blocking the expansion of slavery into the West. In so doing, Wilmot enraged the South, split the Democratic majority along sectional lines, and infuriated President Polk, who wanted to avoid the slavery issue altogether while obtaining as much territory as possible. Though ultimately defeated in the Senate, the Wilmot Proviso would be re-introduced by the Whig majority in the House during each session of Congress for the next four years, provoking a heated sectional debate that many historians have recognized, in retrospect, as a pivotal precursor to the Civil War.
At the time it was first proposed, the Wilmot Proviso offered a means by which northern Whigs could appear supportive of national troops while withholding their endorsement of the Mexican-American War. Many northern Democrats found the proviso appealing because it would enable them to embrace the popular concept of free labor in the process of acquiring any conquered territories. Many white voters envisioned free labor as a means of earning a decent and respectable wage in the emerging capitalist system, without the presence, interference, or competition that slavery represented. The Mexican War seemed to reinforce partisan allegiances on both sides of the political spectrum, for Democrats defended the conflict as justifiable while most Whigs railed against it. Yet this war sparked issues that continually threatened to undermine the national parties by realigning their members according to their sectional interests. At the time, a sufficient number of southern senators could join together to defeat the Proviso, thus uniting pro-annexationist Democrats (mostly southerners) with the anti-annexationist Whigs (mostly northerners). The southern contingent of both parties joined forces to fight any attempt to block slavery's expansion into new states. Northerners, in turn, resented the institution of slavery and the three-fifths compromise that enabled southern congressmen to exert an influence disproportionate to the actual number of voters in their respective constituencies. (Because of the three-fifths compromise, the slave population bolstered the voting power of southern whites.)
As historian Charles Sellers has argued, "what Wilmot called the 'White Man's Proviso' signaled a fusion of antislavery with racism that proved unstoppable." In other words, politicians who claimed to stand for "free soil, free labor, and free men" could derive political popularity by standing against the expansion of slavery on behalf of white men.9 Abolitionists—motivated by a sincere desire to help enslaved blacks—remained a small minority in the North, but working-class whites who cared little for the fate of black people increasingly came to oppose the institution of slavery because they viewed it as a competitive threat to their own jobs and an institution that undercut and demeaned the value and honor of free labor.