The Debate on the Wilmot Proviso

Although James K. Polk won a Democratic Congress, that doesn't mean every lawmaker was on the same page. There were two sub-groups within the Democratic Party: the Northern Democrats and the Southern Democrats. Each had their own viewpoints and issues regarding the party, and had different expectations of the other group.
The Wilmot Proviso was not Wilmot's own idea, but was instead drafted during the Barnburner strategy meeting before Congress was called into session. Wilmot was chosen to present the amendment to Polk's appropriations bill because he was close to many southerners, so he would have no trouble gaining the floor during the House debate.

The Birth and Death of the Wilmot Proviso

Initial Opposition in the House

William Wick, a Democrat from Indiana, proposed an alternative to the proviso: that the Missouri Compromise line just be extended to the Pacific coast.

This counter-proposal was defeated 89-54.

House Voting

A vote was then called to add the proviso to Polk's original appropriations bill.

This passed 83-64.

Southern Democrats called a vote to table the entire bill in an effort to kill the proviso.

This was defeated 94-78.

A final vote to approve the bill, with the proviso, and send it to the Senate.

Passed 85-80.

The outcomes of these votes largely fell along sectional lines, not party lines.

Deliberation in the Senate

The Southern Democrats had more power in the Senate than in the House, given the larger population of the Northern half of the country. The goal of the Southern Democrats here was to reject the proviso and send the original bill back to the House for approval.

To keep the proviso intact, John Davis, a Whig from Massachusetts, held the floor to stall for time and prevent a vote from being called for the proviso's rejection. However, he ended up filibustering for too long, the Senate adjourned and Congress went out of session.

Resurfacing of the Issue

The next time the Wilmot Proviso would be deliberated over would be the next year, after Polk renewed his request in the appropriations bill. Reintroduced by New York Democrat Preston King, the proviso's bans on slavery were expanded to any new territory gained thereafter, not just the ones from the Mexican Cession.

Stephen King, a Democrat from Illinois, reintroduces the alternative of an extension of the Missouri Compromise line. Defeated 109-82.

The bill then passed the House with proviso, but passed the Senate with the proviso rejected. The House then approved the original bill sans proviso as 22 "doughfaces" voted with the South, and the proviso had been defeated.

The proviso was again proposed as a rider to the Treaty of Guadalupe Hidalgo, but this died in the Senate as well. The Southern Democrat's logic for rejecting the Wimot Proviso was based on the concept of popular sovereignty. This would become the Southern Democrat's response to the Wilmot Proviso. As described in a letter between two Southern Democrats:

“Leave it to the people, who will be affected by this question to adjust it upon their own responsibility, and in their own manner, and we shall render another tribute to the original principles of our government, and furnish another for its permanence and prosperity.”