Introducing the Wilmot Proviso
Upon taking his seat in Congress, Wilmot initially supported the policies of Democratic president James K. Polk. He also became part of an informal group of anti-slavery Democrats led by New York Congressman Preston King, who was a protégé of New York 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. As James G. Blaine later wrote:
"David Wilmot represented a district which had always given Democratic majorities and was himself an intense partisan of that political school. He was a man of strong physique and strong common sense; of phlegmatic temperament, without any pretension to genius; a sensible speaker, with no claim to eloquence or oratory. But he had courage, determination, and honesty".
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 House. 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."
Wilmot modeled the language for what would usually be referred to as the Wilmot Proviso after the Northwest Ordinance of 1787. 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". 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.
In a February 1847 debate over the Proviso, Wilmot explained that he was not an abolitionist, and was not seeking to abolish slavery in the Southern states, but simply wanted to preserve the integrity of free territories that did not have slavery and did not want it:
"We ask that this Government protect the integrity of free territory against the aggressions of slavery—against its wrongful usurpations. Sir, I was in favor of the annexation of Texas. I supported it with the whole influence which I possessed, and I was willing to take Texas in as she was. I sought not to change the character of her institutions. Texas was a slave country. We voted for the annexation of Texas. The Democracy of the North was for it, to a man. We are for it now—firmly for it. Sir, we are fighting this war for Texas, and for the South. I affirm it; here is a matter well known to the Union. Now, sir, we are told that California is ours; and so it is. I intend to refer more particularly to this subject before I conclude. But, we are told, California is ours. And all we ask in the North is, that the character of its territory be preserved. It is free; and it is part of the established law of nations, and all public law, that when it shall come in to this Union, all laws there existing, not inconsistent with its new allegiance, will remain in force. This fundamental law, which prohibits slavery in California, will be in force; this fundamental law, which prohibits slavery in New Mexico, will be in force. Shall the South invade it? Shall the South make this Government an instrument for the violation of its neutrality, and for the establishment of slavery in these territories, in defiance of law? That is the question. There is no question of abolition here, sir. It is a question whether the South shall be permitted, by aggression, by invasion of right, by subduing free territory and planting slavery upon it, to wrest this territory to the accomplishment of its own sectional purposes and schemes? That is the question. And shall we of the North submit to it? Must we yield this? It is not, sir, in the spirit of the compact; it is not, sir, in the Constitution."
In an 1848 speech, Wilmot responded to critics who called him a radical abolitionist by pointing to Thomas Jefferson's proposed Land Ordinance of 1784, which would have banned slavery in a large portion of western territory slated for federal expansion.
"This sir, looks very much like the 'Proviso.' Here is the original "firebrand"—the heresy, for holding onto which men are now proscribed by the government of their country. Mr. Jefferson, had he lived at this day, would have been denounced as an abolitionist, and a disturber of the peace of the Union."
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". The U.S. Senate adjourned rather than approve the bill with the proviso.
