For obvious reasons, most of my attention has been on the Republicans and Donald Trump because Clinton has had pretty much a lock on this since she scared away any potentially legitimate challenger.
Sanders is interesting, though, for what he might do in a different year. In 1964, the Republicans expected to lose, and had an internal debate over whether to at least try to win with Rockefeller, or run an aspirational candidate like Goldwater, who couldn't win, but might move the party. They went the latter route, and Goldwater did move the party, paving the way for Reagan.
Sanders' original goal was to move the party to the left, and while he might have convinced himself that he has a chance to win now, he doesn't. And since 2016 is a good bet for the Democrats, it is a bad time to nominate an aspirational candidate who might lose an election that the Democrats should otherwise win. That makes him actually an irritant to the very people Sanders wants to influence. In a bad year for Democrats, Sanders might appeal for the same reason Goldwater did.
Of course, the rules are different now. Goldwater had to convince a nominating convention, and Sanders would need to convince voters, but the underlying logic is similar.
The problem is recognizing when your party is just toast. The Republicans could have done that in 2008, and instead of an aspirational candidate, they went with a relatively strong general election candidate they didn't really like, and who was destined to lose anyway.
From a strategic perspective, it is important to think about your overall chances in a given year.