Senator Bernie Sanders vowed on Sunday to fight on after losing the Nevada caucuses, predicting that he would pull off a historic political upset by this summer’s party convention.

But the often overlooked delegate count in the Democratic primary shows Mr. Sanders slipping significantly behind Hillary Clinton in the race for the nomination, and the odds of his overtaking her growing increasingly remote.

Mrs. Clinton has 502 delegates to Mr. Sanders’s 70; 2,383 are needed to win the nomination. These numbers include delegates won in state contests and superdelegates, who can support any candidate. She is likely to win a delegate jackpot from the overwhelmingly black and Hispanic areas in the Southern-dominated Super Tuesday primaries on March 1, when 11 states will vote and about 880 delegates will be awarded.

Since delegates are awarded proportionally based on vote tallies in congressional districts and some other areas, only blowout victories yield large numbers of delegates. And Mrs. Clinton is better positioned than Mr. Sanders to win big in more delegate-rich districts, like those carved out to ensure minority Democrats in Congress, where she remains popular.