why does atticus think he will not win the case? why does he take the case of tom robinson anyway?

Atticus knows he is up against generations of bigotry and racism. The hate is so ingrained in much of the white population that winning this case seems remote. The sexual nature of the case also doesn't help his chances. Few whites would take the word of a black man over a white accuser, even if she is a Ewell. Atticus takes the case because it is simply the right thing to do. Tom deserves a defence, he also happens to be innocent. Atticus tells Scout that if he didn't act according to his conscience, "I could never ask you to mind me again."