the majority of people DON'T think he's too wing nutty... that's just Clinton fear mongering... there's literally no data anywhere that suggests that what you're claiming is true. Sure even some GOPer are saying that they'd vote for Sanders over Trump, who IS seen as a nut... they only chance Trump has is if the establishment holds their nose and votes for him, and the only candidate that can energise them to do that is the boogeyman of the right, Hillary Clinton...
And when you consider just how completely untrusted - for very good reason - Hillary is, she'd drive down the Dem vote.
Sanders on the other hand energises, provides a sane alternative to Trump and is seen as trustworthy by even many Republicans.
So just saying that he's a retread of past defeats isn't good enough.... you'll need to find some data to back that up. And you can't, because it doesn't exist.