The Mexican War was less than three months old when the seeds of a new conflict began to sprout. On August 8, 1846, a sweltering House of Representatives reassembled to clear its calendar for adjournment. A freshman Democrat from Pennsylvania, David Wilmot, delivered a provocative speech. He favored expansion, Wilmot explained, even the annexation of Texas as a slave state. But slavery have come to an end in Mexico, and if new territory should be acquired, “God forbid that we should be the means of planting this institution upon it.” Drawing upon the words of the Northwest Ordinance, he proposed that lands acquired from Mexico, “neither slavery nor involuntary servitude shall ever exist in any part of said territory.”

Proposed as an amendment to an appropriations bill, the Wilmot Proviso was never a law, but it politicized slavery once and for all. For a generation, since the Missouri controversy of 1819-1821, the issue had been lurking in the wings, kept there most of the time by politicians who feared its disruptive force. From that day forth, for two decades the question would never be far from the center stage.

The House immediately adopted the Wilmot Proviso, but the Senate refused to concur. When Congress reconvened in 1846, Polk prevailed on Wilmot to withhold his amendment, but by then others were ready to take up the cause. When a New York congressmen revived the proviso, he signaled a revolt by the Van Burenites in concert with the anti-slavery forces of the North. Once again the House approved the amendment. Once again the Senate refused to do so. In one form of another Wilmot’s idea kept cropping up. Abraham Lincoln later recalled that during one term as congressman, 1847-1849, he voted for it “as good as forty times”.

John Calhoun meanwhile devised a thesis to counter the proviso and set it before the Senate in four resolutions on February 19, 1847. The Calhoun Resolutions, which never came to a vote, argued that since the territories were the common possession of the states, Congress had no right to prevent any citizen from taking slaves into them. To do so would violate the Fifth Amendment, which forbade Congress to deprive any person of life, liberty, or property without due process of law, and slaves were property. By this clever stroke of logic, Calhoun took that basic guarantee of liberty, the Bill of Rights, and turned it into a basic guarantee of slavery. The irony was not lost on his critics, but the point became established southern dogma. – echoed by his colleagues and formally endorsed by the Virginia legislature.

Senator Thomas Hart Benton of Missouri, himself a slaveholder but also a Jacksonian nationalist, found in Calhoun’s resolutions a set of abstractions “leading to no result.” Wilmot and Calhoun between them, he said, had fashioned a pair of shears. Neither blade alone would cut very well, but joined together they could sever the ties of the union.