"Bernie Sanders isn't electable" is the criticism constantly leveled against Bernie Sanders (and Donald Trump for that matter).
Electability is a myth. Saying a candidate is unelectable doesn't make it so. The very idea of electability is a product of the undemocratic party system that has developed in the United States, in which politics is a zero sum game, and the game must be won at all costs. In effect it is entirely about control, and not about the interests and desries of the electorate.
Anyway, the so-called front-runner in the 2016 election is Hillary Clinton. But why should we assume she's electable? She was the favorite going into 2008 and couldn't get nomination. Now she's getting beat by a socialist! As our friend Matt says, "'Electability' is code for I'm afraid, I just want the familiar status quo person because I see them on TV a lot and people know who they are."
You know a good pretty good way to figure out if someone is a viable candidate or not? Hold an election!