Question

Why were southern politicians opposed to the Wilmot Proviso?

Answer 1

The proviso would limit the spread of slavery.

Answer 2

The correct answer is the Wilmot Proviso would stop the spread of slavery into new territories gained by the United States.

This idea, introduced by David Wilmot, would stop slavery from expanding into the new territories the US gained after the Mexican-American War. In the Treaty of Guadalupe Hidalgo, the US received modern day California, New Mexico, Arizona, Colorado, Utah, etc. Instead of passing the Wilmot Proviso, Congress enacted the Compromise of 1850. This would be another law that just delayed a long term solution to the institution of slavery in the US.