Unless Sanders comes within a few points (~5% maximum) if he loses the nomination to Hillary, I can't see him attempting a 3rd party run.
The silent (although not as silent as one might think) majority of Sanders supporters would vote for Hillary if he loses the nomination.
There must be some statistics somewhere on notable 3rd party presidential candidates when it comes to party nominations they lost, meaning numbers of democrats who lost their party's nomination and went 3rd party versus republicans who lost their party's nomination and went 3rd party. I want to say that it's more often republicans, but it could actually be an even split.
In any case a 3rd party run has never been even remotely successful. I don't see this election being the groundbreaking time where a 3rd party run comes even close to getting double digit electoral votes, never mind actually coming in close to winning. Besides in 24 (I might be wrong on this number but it's around there) states the electoral votes can go for whomever they want, regardless of popular votes or otherwise.
The whole system is pretty defunct at its core to begin with to think some magical 3rd party candidate or even a non-mainstream candidate has a chance to win. Unless you're in a swing state like Ohio, Virginia, or Florida (a few others too) your vote is pretty pointless anyway. Good luck getting California to ever have its electoral votes cast for a republican candidate, for instance.
The whole get out and vote thing, rock the vote, I voted, and so on gimmicks are just feel good moments to make people feel like they matter.
The Democrats had their turn with the WH, Senate, and HofR and now the pendulum is swinging red so just start getting used to a republican president at the end of this year. The big question is who. Bush was supposed to be the guy and still might be. Trump would be funny, but I don't see it happening. Rubio, seems too young, but statistically has shown to hold up against everyone in every match up for awhile now.
It still sounds a little odd to me, President Rubio. Maybe it's just because he's a little young (age 44), but that's where all the numbers point to.
Nothing is 100% though, so it's definitely entertaining to watch.