We often hear about how left-wing candidates can “never win in the general” because they’re “too far outside the mainstream.” Instead, we need candidates who can “appeal to the center.” And this may well be true — as long as you hold the electorate constant. Bernie Sanders probably is a little too extreme for the ideological “center” of the declining number of people who show up to ratify the depressing mediocrity that the major parties serve up, just as Jeremy Corbyn is likely to turn off those who relish the opportunity to choose between Tory Classic and Tory Lite.
One unique property of someone who is “outside the mainstream” in those terms, though, is that they can appeal to people who usually don’t bother to vote. We know that this works because it has literally happened in both of the most recent presidential elections, where the “unelectable” Barack Obama — a black man, with the middle name of Hussein, with Muslim family background, with ties to a radical black preacher who declared “God damn America,” etc., etc., etc., etc. — managed to get elected by reaching out to a good chunk of the people who have no time for the uninspiring products of the “rush to the center” strategy.
If he was running only within the 2000 or 2004 electorate, I have no doubt he would have been destroyed. But in a country with low voter turnout, you also have the option of creating your own electorate, which is what Obama effectively did. And I daresay that the left has more room to generate fresh voters than the right does, as evidenced, for instance, by the fact that it’s the right that’s trying to suppress voter turnout.