The Wilmot Proviso and the policies it sought to make legal are are an extension of the oppressive conditions that permeated the southern region of the United States and ultimately led to Civil War. I believe that Northern politicians were well aware of the modus operandi that ole David Wilmot was operating in conjunction with when he crafted his legislation. How would Northerners know if the voting process was rigged or that blacks were not allowed to vote? Dave gave them the rope-a-dope. However, rather than continue to get slugged, Northerners balked at his ideas and southerners did too. I imagine skeptical Southerner thought "what of the negroes get to vote?" While Northerners wouldn't believe anything except currency.
I believe that this legislation connects with our reading by establishing a social milieu for Whitman. Whitman's consistent and persistent editing of the texts reflects the changing environment that he experienced. For me, Song of Myself represents a return to all things "Americana." Even the slave, who picks cotton, has a role to play and will be nursed to health and sent on his way by wonderful Walt Whitman.