Assuming Clinton gets the Democratic nomination, I was looking into whether or not Sanders could run as an independent in California (the way John Anderson did in 1980).
California Elections Code Section 8301 says, “A candidate for whom a nomination paper has been filed as a partisan candidate at a primary election, and who is defeated for his or her party nomination at the primary election, is ineligible for
nomination as an independent candidate,” which makes it sound like that whichever of Sanders/Clinton/Trump/Cruz doesn’t get nominated can’t then run as an independent in California.
On the other hand, the rest of the Election Code says that the office being run for is “presidential elector” and not “president”, and sections 8303 and 8304 say:
“Whenever a group of candidates for presidential electors,
equal in number to the number of presidential electors to which this
state is entitled, files a nomination paper with the Secretary of
State pursuant to this chapter, the nomination paper may contain the
name of the candidate for President of the United States and the name
of the candidate for Vice President of the United States for whom
all of those candidates for presidential electors pledge themselves
to vote.
When a group of candidates for presidential electors
designates the presidential and vice presidential candidates for whom
all of the group pledge themselves to vote, the names of the
presidential candidate and vice presidential candidate designated by
that group shall be printed on the ballot pursuant to Chapter 2
(commencing with Section 13100) of Division 13.”
Notice that it does not say that the “nomination paper” need have the approval of the candidates themselves.
Which brings up the original question: could Sanders supporters press to get him on the ballot in November in enough states even if he says he doesn’t want it? Of course, there will be calls of, “But this would just split the Democratic vote!,” but the response would probably be, “Clinton would do better in states that Trump would win anyway, and Sanders would win most of the other states thanks to the independent voters.”