He is best known for being the prime sponsor and eponym of the Wilmot Proviso, a failed proposal to ban the expansion of slavery to western lands gained in the Mexican Cession.

A northern Democrat when he introduced and supported the Proviso, he subsequently became a notable member of the anti-slavery Free Soil Party.

[2] Upon taking his seat in Congress, Wilmot initially supported the policies of Democratic president James K.

[4] 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.

[5]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.

[7] 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".

[8] Historian Sean Wilentz writes that it is unclear why Wilmot, an "unremarkable" first-term Congressman, was the one to introduce the measure.

[6] 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.

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?

"[9]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 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".

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.

[13] 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.

"[15] 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.

[3] Wilmot was President Judge of the Pennsylvania Court of Common Pleas for the Thirteenth Judicial District from 1851 to 1861.

[citation needed] Wilmot was elected as a Republican to the Senate to fill the vacancy caused by the resignation of Simon Cameron.

[2] He was a member of the Peace Convention of 1861, held in Washington, D.C., in an effort to devise means to prevent the impending American Civil War.

[2] Wilmot was nominated by president Abraham Lincoln on March 6, 1863, to the Court of Claims, to a new seat authorized by 12 Stat.