TL;DR: Bernie Sanders isn’t a leftist pipe dream so don’t support him. Also, once he inevitably loses, it’ll make us all very sad.
Meanwhile, the mantra that he can’t win is getting old and tired:
Bernie Sanders shocks with 41 percent of the vote:
http://www.politico.com/story/2015/06/wisconsin-straw-poll-surprise-a-narrow-clinton-win-118727.html
The Vermont senator’s ideas and policies have far more public support than our oligarchs want you to believe: