Anti slavery because it wanted to ban slavery in the new land from the Mexican cession (i.e. the land the US acquired from the Mexican American War). However, it did not become law because not enough of congress approved probably because it was too progressive and anti slavery for the time. It went against the Missouri compromise (i.e. a law that banned slavery above a certain longitude and allowed slavery below that longitude) since a good portion of the Mexican extended below the slavery longitude. Also, many southerners wanted to expand slavery into the new territory for both economic and political reasons.
