The South did not take the Wilmot Proviso laying down, though the fire-eaters’ answer doesn’t receive near the same coverage in most history books.
William Lowndes Yancey of Alabama served only a term and part in the House but won some notoriety for his forceful oratory in favor of Texas annexation and the Mexican War, which provoked an opponent to challenge him to a duel. During his time in Washington, Yancey became increasingly convinced that a Northern conspiracy aimed to destroy the South. His speeches accordingly gave Northerners reason to suspect the same of the South towards the North. Yancey served only a term and a portion before resigning for financial reasons and to avoid from Northern Democrats who he saw as traitors.
But Yancey did not leave politics behind. Alarmed by the Wilmot Proviso, he saw in the nomination of slaveholder and war hero Zachary Taylor by the Whigs a vehicle to restore the old bipartisan status quo on slavery. Taylor had only to reject the Proviso. Taylor refused. That left Yancey with his own Democrats and he went to the Alabama Democratic convention with a mission and succeeded in getting it to adopt a series of resolutions that came soon called the Alabama Platform. Among them:
Resolved, That this Convention pledges itself to the country, and its members pledge themselves to each other, under no political necessity whatever, to support for the offices of President and Vice President of the United States, any person who shall not openly and avowedly be opposed to either of the forms of excluding slavery from the territories of the U.S. mentioned in the resolutions, as being alike in violation of the constitution, and of the just and equal rights of the slaveholding States.
Resolved, That these resolutions be considered as instructions to our delegates to the Baltimore Convention, to guide them in their votes in that body; and that they vote for no men for President or Vice-President, who will not unequivocally avow themselves to be opposed to either of the forms of restricting slavery, which are described in these resolutions.
The Alabama and Georgia legislatures endorsed the platform, as did party conventions in Florida and Virginia. The Baltimore convention of 1848, however, rejected the platform and selected Michigan’s Lewis Cass, originator of the concept of popular sovereignty (That the territories, not the Congress, should decide on slavery within their bounds.) as the Democratic nominee.
Yancey and one other delegate from Alabama walked out and the convention proceeded without them. Yancey did not return to party conventions until 1860, when considerably more delegates stormed out with him.