The Wilmot Proviso:

"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."

The above wording was proposed by Congressman David Wilmot on August 8, 1846 as a reaction to then President Polk's announcement he was requesting an appropiation of $2,000,000 for negotiating a settlement with Mexico for the territories gained during the Mexican American War.  While the law was not passed by the Senate, the vote revealed the sentiments of the nation at the time - with those in favor and those opposed in the House of Reprentatives divided not on party, but on sectional lines.  There were three votes taken that day and each passed the House.