Why does Atticus think he can't win Tom Robinson's case?

Atticus mentions to Jim and Scout that the case is sensitive and the whole world is basically against them from the beginning because it's one black man's word against several white people. No matter how upstanding the black man or how unappealing the white people, the whites will always be listened to before the black man's word is considered. Atticus tells Jim and Scout that he isn't just defending Tom Robinson: he's fighting 100 years of history and he is not going to win against all that precedence, no matter how great an attorney he may be.