I don't think negative ads will hurt him.
The only way to play this guy is to catch him in a lie. The silver tongue is good, but it works by initiating a large amount of trust and believability. One thin lie, properly caught and properly handled can break the hypnotic spell, and the whole thing comes crashing down.
But he is slick He knows the lie will hurt him, so he never really says anything, and never gets bolted down. He's gonna be a tough nut to crack.
but nobody really tried to get him yet.