James M. McPherson, Ordeal by Fire: The Civil War and Reconstruction, 3rd ed. (New York: McGraw-Hill, 2001), 65-66.
On August 8, when the war was barely three months old, an obscure first-term Democratic congressman from Pennsylvania named David Wilmot offered an amendment to an appropriations bill: “that, as an express and fundamental condition to the acquisition of any territory from the Republic of Mexico…neither slavery nor involuntary servitude shall ever exist in any part of said territory.” The principle embodied in this amendment – the Wilmot Proviso, as it came to be known – remained the lodestone of sectional conflict for the next fifteen years…Northern Whigs voted unanimously for Wilmot’s proviso; so did all but four Northern Democrats, while every Southern Democrat and all but two Southern Whigs voted against it. Having passed the House, the proviso failed to come to a vote in the Senate at this session. At the next session, in February 1847, the House repassed the proviso; but the Senate with five Northern Democrats joining the Southerners, passed the appropriations bill without the antislavery amendment. Under heavy administration pressure, twenty-three Northern House Democrats then receded from the proviso and cast the necessary votes to pass the bill unamended.