Wilmot Proviso

The Wilmot Proviso was a call to stop slavery in it's tracks. Historically it would have prevented states, acquired from Mexico, from having  slavery. The proviso didn't pass in the Senate, instead a compromise was reached that allowed slavery to still exist in some of the newly acquired states. To Whitman, the passing of the proviso would have meant a large step towards the unification of people. It would have reinforced his belief in the unity and oneness of all men and women.

The fact that the proviso wasn't passed was a big slap in the face for people like  Whitman who believed in the equality of people. The proviso was introduced in 1846, nearly a decade before the publication of Leaves of Grass. This historic event surely had an effect on Whitman and influenced the the theme of unity in "Song of Myself". As a result, it can be assumed that Leaves of Grass wasn't only a poetic work but also a source of revolutionary propaganda, with a call for the liberation of slaves as well as unity of  the States.