Henry Seward expands the antislavery argument beyond the moral appeal of the abolitionist by talking about or addressing the country as a whole and not just about the slaves. He doesn't use the term "slaves" to refer it to them but rather, he calls them as "the laborers", so that, those who are non abolitionist would imagine slaves as laborers or working men. He also once said, as quoted, “the united states must and will, sooner or later, become either entirely a slaveholding nation, or entirely a free labor nation." Hope this answer helps.