UPDATE: The debate is off. Trump has officially backed out. The original blog post is below.
If a Trump-Sanders debate happens, it will turn into a Hillary bash-fest. Trump will use and magnify Sanders’ arguments against Clinton, forcing Bernie to defend her weakly from his own attacks, or to join in. Clinton’s legitimacy would be weakened, Sanders would look bad, and Trump will have taken control of the Democratic primary narrative.
That Bernie floated this idea and is considering it tells us something not very good about either his intentions or his intelligence.
I suspect the reason Bernie wants to do this is that he intends to weaken Hillary, and he thinks it’ll show the superdelegates that they should overturn the will of the majority of Democratic voters and give the nomination to him.
Sanders’ only message now is, “I’m a better candidate against Trump than Clinton is.” That is based on a myth, but that’s the only argument he can make now. He can’t lay claim to the support of most Democrats. He can’t lay claim to the majority of pledged delegates. It’s obvious why he wants to debate Trump. It’s a ploy to “steal” (it’s what Bernie at the beginning of the primaries would have called it) the nomination from Hillary Clinton.