James K. Polk's agenda, unlike that of his two immediate predecessors, was largely driven by foreign policy considerations, namely, territorial expansion and foreign trade. Each of these, however, promised profound domestic consequences, the former in terms of the slavery question and the latter in terms of what to do about tariff levels. In addition, Polk had promised during the campaign to revive the now moribund Independent Treasury system, first enacted in 1840 during the presidency of Martin Van Buren but repealed a year later by a Whig-dominated Congress. Thus, Polk intended to address four of the most contentious issues of the Jacksonian Era, each of which had sectional implications: territorial expansion, slavery, banking, and the tariff. Of these, slavery had been the least debated at the national level; with the Mexican War under Polk, however, this would change.

Walker Tariff of 1846

During his election campaign Polk had stated his opposition to protective tariffs but not to tariffs that might in some way provide protection for certain goods by making imports unaffordable. But at what point does a tariff rate on any given item become protective? To answer this question, Polk soon after his inauguration commissioned a study of tariff levels from his secretary of the treasury, Robert J. Walker. Following his nationwide survey, Walker suggested a much more significant reduction in tariff rates than Polk had intimated to northeastern Democrats he would support. Soon a bill was before the U.S. Congress, despite opposition from within Polk's own cabinet. After a debate along largely sectional lines, with southerners and westerners favoring the new tariff bill and all but a few northerners opposing it, Vice President George M. Dallas cast the tie-breaking vote in the Senate. Polk quickly signed into law what became known as the Walker Tariff. The Walker Tariff moved rates downward towards revenue-only levels and dropped the policy of an ad valorem rate (a percent of the value of the goods) in favor of a set rate regardless of the value. A few commodities were listed as duty-free.

Independent Treasury Act of 1846

With his hard-won victory in the tariff debate, Polk next moved to revive the Independent Treasury Act that President Martin Van Buren had signed into law in 1840 and the Whig-dominated Congress had repealed the next year. The act established independent treasury deposit offices separate from private or state banks to receive all government funds. The system was designed to be a long term replacement for both the Second Bank of the United States, which Jackson had "killed," and a remedy for the ensuing wild speculation resulting from Jackson's policies that contributed to the major depression of the late 1830s. The Whigs had repealed the act as a step toward recreating an institution more similar to the Second Bank of the United States. The new Independent Treasury established by Polk entrusted the federal government with the exclusive management of government funds and required that disbursements be made in hard specie, such as gold or silver, or in paper backed by gold or silver. This would, hopefully, avoid undue speculation in western lands as the nation expanded its territory.

Westward Expansion

The Mexican War (see Foreign Affairs section) progressed too rapidly and too decisively for much antiwar sentiment to gain wide support at home. The most substantial opposition came from New England Whigs, Christian pacifists, and antislavery or abolitionist Americans. What these groups had in common was not so much a rejection of a belief in American's Manifest Destiny. Rather they objected to using military force to spread American republican institutions, rather peaceful annexation. For most opponents, the war was a transparent attempt to extend slavery into new territories that would become new slave states, thus ensuring that the South would control Congress and the presidency into the foreseeable future. In this heated atmosphere, it is understandable that one of the main domestic issues during James K. Polk's presidency was the extension of slavery into new American territories.

Wilmot Proviso: Slavery in the New Territories

On August 8, 1846, Democratic Congressman David Wilmot of Pennsylvania introduced an amendment to an Army appropriations (spending) bill that brought the slavery issue to a head. The "Wilmot Proviso" asserted that "neither slavery not involuntary servitude shall ever exist in any part of" the territory acquired from Mexico. Wilmot's amendment passed in the House but failed even to come to a vote in the Senate. Whether the amendment won approval or not was less consequential than the fact that the debate over the Wilmot Proviso revealed a growing sectional rift within the two major parties. While nearly all northern Democrats joined with all northern Whigs to support the amendment, southern Democrats and southern Whigs voted against it. "Mr. Polk's War" had clearly exposed the old wound of sectionalism, and this time it threatened to split even the reliably pro-slavery Democratic Party.

The issue also weakened Polk with his most avid southern supporters, who rallied around John C. Calhoun of South Carolina. Calhoun had formulated "the southern rights position" to the Wilmot Proviso, introducing resolutions in the Senate affirming the rights of slave owners to transport their "human chattel" anywhere in the territories of the United States. Polk, on the other hand, endorsed the idea of extending the old Missouri Compromise line of 36°30' to the Pacific Ocean. This would have excluded slavery from present-day Washington, Oregon, Idaho, Utah, Nevada, and the northern half of California while allowing it in present-day New Mexico, Arizona, and southern California. At the end of Polk's term, the issue of slavery in the new territories loomed as one of the key issues facing the nation.