Maybe it's because their motives are too obvious, or that people are so attached to Trump that nothing will shake their support, but I just don't see the negative attacks working against him in a significant way.
OTOH, his negatives are already high, so I think he has reached his ceiling, as long as their are other true conservatives in the race.
In addition, the attacks may fail for two other reasons: (1) Trump can defend his positions with attacks of his own and (2) The funding for this effort is a clear demonstration that the attacker is going to be in dept to someone for this payment. Thus, the Donald remains the only candidate who is not bought.