We've written about the possibility that Trump might go 3rd party spoiler and cost the GOP its logical majority as the pendulum swings back to the right. Nobody seems to consider that Bernie Sanders might also choose to go 3rd party if, as expected, Clinton gets the nomination. He could cost her the majority she hopes for.
Trump and Sanders each speak to members of their party's grand coalition who feel ignored by the party's mainstream candidates. Either or both could conclude they represent too many serious, well-intentioned-but-otherwise-unrepresented voters to drop out if not chosen as nominee, leading to a 3rd party run.
Third party candidates don't get elected president in the U.S., at least so far. It doesn't seem to keep people from running as spoilers, perhaps they enjoy sticking it to whomever their major party nominates, costing them the election. There's a certain satisfaction in doing that.
