There is a presumption amongst Sanders supporters that he'll easily win in debates with Clinton. No doubt, he's a quick thinker with unique ideas, while Clinton is more measured and parochial. That said, it's naive to think that substance plays a significant role in the public's perception of Presidential debates.
After all, we have Kennedy and Nixon as empirical evidence that visceral reactions determined peoples' perceptions as to a winner: Nixon won, according to those who heard the debate on the radio, whereas Kennedy won to those who saw it on television.
Perceptually, Sanders will have to overcome (possibly unfair, but real) disadvantages debating Clinton. First, and obviously, he will have to belie his nature and be considerably passive. Rick Lazio can tell a tale about that. Secondly, he has an unfortunate habit of pointing. A lot. He also grimaces. A lot.
Clinton, on the other hand, is trained to be poised. Of course she'll lash out, as she did with Obama, but that's actually (and, again perhaps unfairly) to her advantage.
Can Sanders overcome the spectacle of a grumpy, pointing, aggressive guy picking on a woman? As I noted, I hate to sound facile, but there are simple realities associated with the art of politics and unique ones as concern debates.
Can he actually "win" a debate with Clinton? If so, how?