The political revolution Bernie Sanders hoped would sweep him into the White House has run into the cold, hard realities of math: He needs to win roughly 63% of the remaining unallocated Democratic delegates to overtake Joe Biden and win the presidential nomination.

To capture that many delegates, the Vermont senator would likely need to notch a victory in every state by significant margins because Democrats award delegates proportionally rather than using a winner-take-all system. That means the former vice president would...