On what single issue does Atticus build his case in To Kill a Mockingbird?
Atticus based his defense of Tom Robinson primarily on the fact that Mayella Ewell must have been beaten by a man who used his left hand, and Tom's crippled left arm and hand ruled him out. Tom's left arm, which was injured in a cotton gin accident when he was younger,
... was fully twelve inches shorter than his right, and hung dead at his side. It ended in a small, shriveled hand...
Sheriff Tate had testified that Mayella's bruises were on the right side of her face, meaning she must have been struck by a left fist; and bruises were found all the way around her neck, undoubtedly caused by the use of two good hands. Atticus further proved that Bob Ewell wrote with his left hand, leaving him, in Atticus's mind, as the prime suspect.
But Atticus pushed home several other issues. Mayella had never been attended to by a doctor, so the act of rape could not be established. According to Tom, Mayella had been the aggressor; she had hugged and kissed him, and when Tom tried to run, she blocked his way. Atticus told the jury that
"She was white, and she tempted a Negro. She did something that in our society is unacceptable: she kissed a black man. No code mattered to her before she broke it, but it came crashing down on her afterward."
Atticus's final point was that it came down to Tom's word against the Ewells'. He tried to convince the jury that all Negroes are not liars, untrustworthy or immoral; and that his word should be believed over that of the Ewells, "whose conduct on the stand" spoke for itself.