Even after Bernie Sanders’s overwhelming popular victory in New Hampshire on Feb. 9, some of his supporters began fretting about a new menace to his candidacy: “superdelegates” who — at least in theory — could deliver the nomination to Hillary Clinton in July’s convention.

Superdelegates are party bigwigs — 712 Democratic leaders, legislators, governors and the like. They can vote for any candidate at the nominating convention, regardless of whether that candidate won the popular vote. These unpledged delegates make up 30 percent of the 2,382 delegates whose votes are needed to win the nomination, and could thus make all the difference.

The status of Hillary and Bill Clinton as senior figures in the Democratic Party has allowed Mrs. Clinton to secure public endorsements from many more superdelegates than Mr. Sanders. Late last year, The Associated Press surveyed 80 percent of the Democratic superdelegates and found that 359 had endorsed Mrs. Clinton, versus eight for Mr. Sanders. The rest remained uncommitted.

In the New Hampshire primary last week, which Mrs. Clinton lost by 22 percentage points, Mr. Sanders won 15 of the state’s 24 pledged delegates, and Mrs. Clinton won nine. But because she has the support of six of the state’s eight unpledged superdelegates, including Senator Jeanne Shaheen and Gov. Maggie Hassan, she is virtually tied with Mr. Sanders in the New Hampshire delegate count.