Wilmot Proviso
In 1848, the Treaty of Guadalupe Hidalgo ending the war was submitted to the Senate for approval. Douglas, now in the Senate, was among those who joined with the South to defeat an effort to attach the Wilmot Proviso to the treaty. In the prior year's debate in the House, Douglas had argued that all of the debate over slavery in the territories was premature; the time to deal with that issue was when the territory was actually organized by Congress. Lewis Cass (Democrat) in December 1847, in his famous letter to A. O. P. Nicholson in Tennessee, further defined the concept of popular sovereignty which would soon evolve as the mainstream Democratic alternative to the Wilmot Proviso: