Donald Trump and Hillary Clinton are the two most unpopular mainstream candidates since records began, as this chart shows:
If Clinton gets the Democratic nomination, as she may well do, Sanders should run as an independent, because he would have a very good chance of winning.
If he does, Hillary supporters would no doubt cry foul, and accuse him of being a spoiler and not a real Democrat anyway. These accusations would be absurd, because:
1. Sanders is much more likely to beat Trump than Clinton is, so it is she who is the spoiler.
2. It’s Clinton, not Sanders, who isn’t a proper Democrat (or democrat): she has accepted $150 million in bribes from big business and Wall Street, and if she is elected it’ll be another four years of America being run for corporations and not ordinary people.
Comparing the three candidates on net favourability (favourably minus unfavourable), we have Sanders +13, Clinton -12, Trump -24:
Note that no candidate with a net unfavourable rating has won, and the only time a candidate with a net favourable rating lost was when both leading candidates had a net favourable rating.
Because Trump and Clinton are both unpopular, Sanders should run. As an independent if need be.