How is Atticus' closing statement in defence of Tom Robinson also an attack on racism? Explain.

Atticus simply states that the prosecution has assumed that because Tom is black he is guilty. Atticus points out that such an assumption is not true since Tom is clearly not guilty. By pointing out the logical fallacies of the state's argument, Atticus is attacking the worst kind of racism.

I think you are referring to this stirring speech,

"I'm no idealist to believe firmly in the integrity of our courts and in the jury system—that is no ideal to me, it is a living, working reality. Gentlemen, a court is no better than each man of you sitting before me on this jury. A court is only as sound as its jury, and a jury is only as sound as the men who make it up. I am confident that you gentlemen will review without passion the evidence you have heard, come to a decision, and restore this defendant to his family. In the name of God, do your duty."

It pretty much speaks for itself. There is only one verdict if this jury can rise above ignorance and racism.