David Wilmot

Upon taking his seat in Congress, Wilmot initially supported the policies of Democratic President James K. Polk.[3] He also became part of an informal group of anti-slavery Democrats led by New York Congressman Preston King, who was a protege of Governor Silas Wright and an ally of former President Martin Van Buren. Although Wilmot opposed the extension of slavery into the territories, he was generally considered to be a Democratic Party loyalist; he supported Polk in the initiation of the Mexican-American War and was the lone House Democrat from Pennsylvania to vote for the Walker tariff. However, during Polk's presidency, anti-slavery Northern Democrats increasingly came to view Polk as unduly favorable to Southern interests. They were specifically disappointed by Polk's decision to compromise with Britain over the partition of Oregon, as well as Polk's veto of an internal improvements bill.[4]

In August 1846, an appropriations bill for $2 million to be used by the President in negotiating a treaty of peace with Mexico was introduced in the United States House of Representatives.[5] Wilmot immediately offered the following amendment:

"Provided, That, as an express and fundamental condition to the acquisition of any territory from the Republic of Mexico by the United States, by virtue of any treaty which may be negotiated between them, and to the use by the Executive of the moneys herein appropriated, 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."[6]

Wilmot modeled the language for what would usually be referred to as the Wilmot Proviso after the Northwest Ordinance of 1787.[6] Unlike some Northern Whigs, Wilmot and other anti-slavery Democrats were largely unconcerned by the issue of racial equality, and instead opposed the expansion of slavery because they believed the institution was detrimental to the "laboring white man."[7] Historian Sean Wilentz writes that it is unclear why Wilmot, an "unremarkable" first-term Congressman, was the one to introduce the measure. Wilmot would later claim that he had introduced the proviso independent of any other members of Congress, while Congressman Jacob Brinkerhoff claimed that he was the true author of the proviso. Wilentz speculates that the proviso was jointly drafted by Wilmot and other anti-slavery Democrats, and that the drafters agreed that whoever had the first opportunity to introduce the proviso would do so.[8]

The House, after first voting down a counter-proposal simply to extend the Missouri Compromise line across the Mexican Cession, passed the proviso by a vote of 83–64. This led to an attempt to table the entire appropriations bill rather than pass it with "the obnoxious proviso attached", but this effort was defeated "in an ominously sectional vote, 78–94".[9] The United States Senate adjourned rather than approve the bill with the proviso.

Wilmot and other Free Soilers sought to exclude slavery from the Mexican Cession (red), which was acquired from Mexico in the 1848 Treaty of Guadalupe Hidalgo.

A measure to the Wilmot Proviso was brought forward at the next session of Congress, with the appropriation amount increased to $3 million, and the scope of the amendment expanded to include all future territory which might be acquired by the United States. This was passed in the House by a vote of 115 to 105, but the Senate refused to concur and passed a bill of its own without the amendment. The House acquiesced, owing largely to the influence of General Lewis Cass.[6] As the 1848 presidential election took shape, the Democrats rejected the Wilmot Proviso in their platform and selected Cass as their candidate to run on a popular sovereignty platform. The new Free Soil Party rallied around the Wilmot Proviso, and nominated Martin Van Buren on a platform calling for "No more slave states and no more slave territory."[10]

By 1848 Wilmot was thoroughly identified as a Free Soiler, but, like many other Free Soilers, he did not oppose the expansion of slavery based on a legal rejection of the short-term existence of the institution itself. In a speech in the House, Wilmot said, "I plead the cause and the rights of white freemen [and] I would preserve to free white labor a fair country, a rich inheritance, where the sons of toil, of my own race and own color, can live without the disgrace which association with negro slavery brings upon free labor."[11] Around the same time, however, Wilmot, in a New York speech, spoke of the ultimate demise of slavery when he argued, "Keep it within given limits …and in time it will wear itself out. Its existence can only be perpetuated by constant expansion. … Slavery has within itself the seeds of its own destruction."[12]

Wilmot was presented as the Free Soil candidate for Speaker of the United States House of Representatives in 1849 and was soon at odds with the mainstream Pennsylvania Democratic Party led by James Buchanan. Wilmot was forced to withdraw from the 1850 Congressional elections in favor of the more moderate Galusha A. Grow.[3]