Who are the white moderates King is referring to in the letter?

When King says, "I had hoped that the white moderate would see this need. Perhaps I was too optimistic; perhaps I expected too much. I suppose I should have realized that few members of the oppressor race can understand the deep groans and passionate yearnings of the oppressed race, and still fewer have the vision to see that injustice must be rooted out by strong, persistent and determined action."

Who are the white moderates he is referring to? (I apologize, I do not know what line or chapter it is located in...as my copy does not provide that.)