Here is the real problem for Clark in going negative on one of the other candidates. If he attacks on the right then he will be proving to many skeptical Democrats that he is a Republican masquerading as a Democrat. If he attacks from the left then he hurts his chances of winning the election. Clark, of all the candidates, is most constrained from going negative.
How come the other candidates don't go negative on Kerry? Each one understands that going negative comes at a cost to themselves. Since people don't like negative campaigns the attacker runs the risk of driving people away from Kerry but to one of his competitors while also raising his own negatives. We are a prisoner of having too many viable candidates. By the time there are only two candidates with a chance Kerry will have the nomination sewn up. yech.