Personally I hope he goes to jail. I can't stand the guy and I like when bad things happen to bad people.
Legally, from what I can tell, the issue is entirely motive... If he paid off his mistress and his aide to keep his affair from him wife, then it wouldn't be illegal. But if he collected and used the money to pay off his mistress and aide because he thought it would help him win the election then the money would fall under the rules for campaign contributions, and the amount of money donated by two individuals exceeded the individual limits. And then from there, if he is found guilty of that, they can say he lied to federal investigators and impeded an investigation.
Of course it will be hard to establish motive, although I am sure they will show that he wasn't just hiding it from his wife because she already knew, and his staffers are turning on him and saying he did it to try to win an election,