Atticus says something profound about the Ewells. He says that the "Ewells were members of an exclusive society made up of Ewells." By this he means that Bob, the father of the Ewells, is a dysfunctional man, who does not know how to take care of his family. For this reason, people in the town made exceptions for him and his family. For example, he was allowed to hunt/trap in all seasons, because the people knew that the Ewells needed food; Bob was a drunk.
Atticus makes this point clear:
“It’s against the law, all right,” said my father, “and it’s certainly bad, but when a man spends his relief checks on green whiskey his children have a way of crying from hunger pains. I don’t know of any landowner around here who begrudges those children any game their father can hit.”
From this, we can say that Ewells were in abject poverty, and it was because of Bob. Similarly, the town did not say much when the Ewell children, like Burris, did not go to school. What could they do?
Here is another quote about the Ewells, but this time the place of their residence is given. This is important, because he lived near Tom Robinson.
"The tribe of which Burris Ewell and his brethren consisted had lived on the same plot of earth behind the Maycomb dump, and had thrived on county welfare money for three generations."
Now as for Tom. According to chapter 9, Tom Robinson lived behind the town dump. More importantly, he says that Tom was a good man and he was from a good family. Calupurnia, a reliable person, could vouch for him.
In light of these points, it would seem that Tom would easily win the trial, but to reason this way is to be ignorant about the racism that exists in Maycomb. The townspeople would always side with a white man and his witness against a black man. Atticus knew this and even from the beginning he said that they would lose. But he still wanted to defend Tom, because it was the right thing to do.