I'd pick the first flight to the USA to campaign for him if necessary, no matter if he runs as a democrat or independent/3rd party. But I agree with other posters, that'd hand republicans the election, and Sanders would get all the (undeserved) blame, so not likely.
If he runs against Clinton in the primaries, he'd have an oportunity to explain to Americans what "socialism" actually means, but I find it higly unlikely that he pushes Hillary to the left. Maybe she'd campaign as a more progressive candidate... but, as President, no difference.
This, he should run in the Dem primaries for media attention, debate access and perhaps the ability to snag a state or 2 and make Clinton/other Dems sweat, then "drop out" , and grab the Green or Justice Party nomination (ballot access main benefit) at their conventions to run in the general. He should have enough polling to get into the debates, his volunteer and fundraising core would stay behind him and maybe he has a chance.
