When Judge Taylor appoints him to take the Tom Robinson case, Atticus takes it very seriously. In fact, in chapter nine, he speaks to Scout about not saying the N-word and that this case affects him personally. He continues to say that he couldn't hold his head high in the legislature or as a father if he did not do his best for the case. Basically, Atticus is implicitly saying that due to the racial and social impact the case will take, he will not throw the trial simply to accommodate the white community. With that said, however, he knows that the chances of getting Tom off with an acquittal is highly unlikely.
During Christmas time, Atticus speaks with his brother Jack about the case. He says the following:
"Before I'm through, I intend to jar the jury a bit--I think we'll have a reasonable chance on appeal, though. I really can't tell at this stage, Jack. You know, I'd hoped to get through life without a case of this kind, but John Taylor pointed at me and said, 'You're it'" (88).
When he says that he may have a chance at an appeal, that means that he does not expect to win the case on the first try--Atticus, thus, knows that a white jury won't side with a colored man.
Another time that Atticus recognizes that he is going to lose the Robinson case is when Link Deas says that he has everything to lose by doing it. Atticus responds as follows:
"Link, that boy might go to the chair, but he's not going till the truth's told. . . And you know what the truth is" (146).
Again, Atticus is bound and determined to at least provide Tom with an outstanding defensive fight even though the odds are stacked against him.
Finally, Atticus knew he would lose the case long before it happened, but he discusses the laws surrounding it with Jem after the trial. Jem is shocked that Tom received a death sentence for rape and wonders if it is an appropriate punishment. Atticus tells his son that he isn't against the punishment for the crime so much as he doesn't like how the trial went. Atticus elaborates as follows:
"I mean, before a man is sentenced to death for murder, say, there should be one or two eye-witnesses. Someone should be able to say, 'Yes, I was there and saw him pull the trigger'" (219).
Since Atticus is an experienced attorney, he had these feelings and knowledge before the trial ever took place. He knew that the prosecution didn't have any eye-witnesses to the alleged crime, but that the case would go on as scheduled. Hence, he did his best to prove that the state didn't have any evidence to convict Tom, but again, since they live in the South in 1935, the case took place anyway.