The Wilmot Proviso is a decision available to 
USA as a part of the Civil War event chain.
It requires that USA owns
- Santa Fe
- Alamosa
- Albuquerque
- Las Cruces
- Grand Junction
- San Francisco
- Los Angeles
- Monterey
The first 5 can be colonized right away, while the latter 3 is in 
Mexico owned 
Californian Republic. USA gains cores on all of them when it takes the decisions Manifest Destiny.
It lowers consciousness by 2 in all free states, and non-states, while it raises consciousness by 1 in all slave states. Unlike what the description says, the decision does not have any effect on the provinces require to take the decision, and they can be amitted as both slave states and free states as the player wishes.
Ingame description
Congressman David Wilmot has suggested a bill that will ban slavery in any state acquired from the Mexican Cession. Supporting the proviso will greatly calm northern fears over Slave Power, but also increase Southern fears of abolitionism.