That might not seem like so much, but it’s a huge number because Democrats award their delegates proportionally in each primary. Mr. Sanders, therefore, needed to win the rest of the race by a margin of roughly 57 to 43 — a big problem for a candidate who had been losing by around 57 to 43. Here’s one way to think about how big his deficit is: If all of the 17 contests Mr. Sanders won got to vote again (which is to say, counted twice), he would still trail Hillary Clinton in the pledged delegate count.

Mr. Sanders seems on track to win 43 percent of the vote again tonight. By falling short, his burden in the remaining states grows even greater: He might now need 58 or 59 percent of the remaining delegates, depending on how the results finish.

Mrs. Clinton’s strength in New York, a state Mr. Sanders’s advisers contended they could win, confirms that he is not on a pace to win the remaining states by such a clear margin.

To the extent that Mr. Sanders’s supporters envisioned a path to a majority of delegates, it hinged on the assumption that he would prove strongest in the most heavily Democratic states, like New York and California.

The assumption was not consistent with the results leading to today. Mr. Sanders lost the states most similar to New York or California, like Illinois and Florida, even Massachusetts. In truth, there has been no relationship between Mr. Sanders’s strength and the Democratic leanings of a county. The stronger predictors of Mrs. Clinton’s strength were diversity and affluence — which augured well for her in New York.