This single paragraph, attached to an appropriations bill at the beginning of the war with Mexico, framed the tense national debate over slavery until the outbreak of the Civil War. Penned by, a little-known Pennsylvania congressman named David Wilmot, the proviso united Northern Whigs and Democrats opposed to slavery's expansion in the West. Although it never became law, the Wilmot Proviso provided an ominous sign that the two-party system would not be able to contain the explosive issue of slavery expansion.

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...