Evidence is not necessary, because this is not a criminal or a civil case. It will be decided by the voters and the voters alone, based on whatever information is available at that time.
You can tell me that I cannot convict Cain: this is true. I cannot absolve him, either, nor can anyone. Each voter in the GOP primaries will decide,and collectively Cain will win or lose.
All I am saying is that my opinion is that he will lose, because based on other similar incidents, those who stonewall tend to lose. One day they say that they are 1000% firm and unyielding in their position, the next day they collapse and resign. Trent Lott did this, the Newtster did this, Agnew did this, Nixon did this. Cain is quite likely to do the same thing. If not, he will simply lose a primary or two and withdraw, and since he is already 65, he will retire of perhaps go back to giving motivational speeches and we will not hear from him again.
Cain is the one who must prove his innocence, because, again, this is the court of public opinion, not a court of law and different rules apply.