Somehow I dont think so. Maybe Im biased cause im predicting Cain to be the winner of the primaries this season. But my impression is that all he has to do is survive the initial onslaught of reporting on this and then he will be fine.
For one, its not a current allegation. its from over a decade ago. unless opposition can make this a pattern of behavior its not going to mean much.
and also. from what i can see, most of the republican electorate actually hates the "liberal media" and they will just see this as an attempt to slander him cause hes ahead. if it cant be proven that he did anything he might actually come out of this more popular than before.
Id think it would be more likely that Cain falls in the poles because of his stumbling on abortion than on this.