Last month I modeled the remaining primaries in the Democratic contest for the nomination. My model represented not what I expected the outcomes to be, but how quickly Senator Sanders would have to attract supporters away from Secretary Clinton in order to finish the primary season 200 delegates ahead. I assumed that if the two candidates were about even in pledged delegates, the superdelegates would maintain their strong preference for Secretary Clinton, but that if Senator Sanders finished with a substantial lead in pledged delegates, enough superdelegates would switch their preference to nominate him.
Ten primaries and one small caucus (Guam) have been held since I modeled the contest. In those 11 contests, Senator Sanders won about 50 fewer pledged delegates than my model required. At this point, he has 1488 pledged delegates and 781 remain to be chosen, so if he won every remaining pledged delegate he would have 2269 pledged delegates, 40 short of the 2309 required for the nomination. (In the unlikely event that he wins all the remaining delegates, the superdelegates would undoubtedly nominate him.) Secretary Clinton has 1767 pledged delegates and needs 542 more pledged delegates (69% of the remaining pledged delegates) to win the nomination without superdelegates.
With the additional 11 contests now decided, what is required for Senator Sanders to win the nomination? In my view he must not only equal Secretary Clinton's pledged delegate count, but must surpass it by a wide margin. I've reduced the victory condition to be that he finishes with at least 150 more pledged delegates than Secretary Clinton, which may be enough of a margin for him to sway the superdelegates. He's currently 279 delegates behind, so to finish 150 delegates ahead he must win 605 of the remaining 781 delegates -- 77% of the remaining delegates. He would then finish with 2093 pledged delegates and she would have 1943.
If he wins 60% of the delegates in all remaining states except California (unlikely but possible) he will receive 144 delegates, and will need to win 451 delegates in California. The hitch is that California sends only 475 pledged delegates, so to win 451 of them he will need to hold Secretary Clinton to less than the 15% threshold that is required to receive delegates. That is so unlikely as to be effectively impossible. If he were to win 75% of the California vote and 60% of the vote in the other remaining states, he will finish the primary season with 1989 pledged delegates, she will finish with 2047 pledged delegates, and the superdelegates will rightly cast their votes to nominate Secretary Clinton.
Barring a catastrophic event, it is no longer possible for Senator Sanders to win the nomination.