There is no such thing as debating with Donald Trump, since what he does is not debating but an exercise in egomaniacal bullying.
It is not a conversation, but a circus act, and everyone else on stage is nothing but a straight man in Trump's performance. Bernie will find himself subjected to a combination of belittling derision, and the constant suggestion that Bernie run as a third party candidate, because Hillary is so bad. The result:
A win for Trump, who will get another chance to perform in front of a national audience.
A loss for Bernie, who will be made to look like a total fool.
And a loss for Hillary, who will end up being subjected to a nationally televised evening of vicious, lying abuse, with Bernie thoughtfully providing a whole new audience for Trump's disgusting vaudeville act, some of whom will inevitably fall for it. On the other hand, not one single Trump supporter could possibly be persuaded to switch to Bernie, because they want hatred and racism, not constructive policies.
This should be the final piece of evidence to convince everyone that Bernie Sanders has run totally off the rails, and is so hungry for victory that he has thrown out every thing he claimed to believe in, in return for a microscopic chance to win the Democratic nomination. This has been a degraded performance, from which, I predict, Bernie will never recover.
Update: Trump today, announcing that he will not debate Bernie:
"Based on the fact that the Democratic nominating process is totally rigged and Crooked Hillary Clinton and Deborah Wasserman Schultz will not allow Bernie Sanders to win, and now that I am the presumptive Republican nominee, it seems inappropriate that I would debate the second place finisher."
There you go. Democrats can only win by cheating, Bernie is a loser and Hillary is a crook. A message brought to the nation thanks to the dedicated work of Trump campaign surrogate Bernie Sanders. Good job there, Bernie.