In this speech, Emerson denounces both politicians and ordinary people. He says that they are both complicit in the evils of slavery and that they have shown their lack of moral fiber.
Emerson is speaking because of the fact that some fugitive slaves had been taken from Boston and returned to slavery. He is aghast that such a thing could have happened. He argues that both ordinary people and politicians accepted this because it was in their narrow and immediate interests to do so. As Emerson says, for example, that
The popular assumption that all men loved freedom, and believed in the Christian religion, was found hollow American brag ; only persons who were known and tried benefactors are found standing for freedom : the sentimentalists went down-stream.
He is saying that people talked about loving freedom, but when it came down to it, they really only cared about themselves.
Emerson says that the support for the Fugitive Slave Act shows that Americans, both politicians and ordinary people, are not nearly as good as they like to say they are. Instead, they are selfish enough to act in ways that make them no better than supporters of slavery.