The circumstances of Mr. de Blasio’s endorsement offered a sign of the reduced interest in his presidential preferences since he initially denied Mrs. Clinton his blessing in April. When the Clinton campaign issued an official announcement about its latest wave of endorsements, Mr. de Blasio’s was given fourth billing, after the mayors of Chicago, Houston and Philadelphia.

By this week, Mr. de Blasio’s six-month delay in issuing an endorsement was viewed in political circles as quixotic at best. Virtually all leading Democrats in New York have already thrown their support behind Mrs. Clinton, along with leading liberal Democrats like Senator Sherrod Brown of Ohio.

But the mayor’s vow of neutrality was initially taken as a liberal line in the sand, part of Mr. de Blasio’s effort to kick-start a national movement to nudge presidential contenders leftward. His public vacillation started in political prime time with a bold declaration during an interview on NBC’s “Meet the Press” — “I want to see a vision,” he said — hours before Mrs. Clinton formally began her candidacy.

For months, the mayor’s allies objected to accusations that Mr. de Blasio wanted to play kingmaker, saying that he simply wanted to hear more detail about the policy plans of presidential contenders, particularly in the early stages of the contest.

More recently, however, many members of the mayor’s inner circle had grown frustrated with his delay, saying that the uncertainty had become a distraction. The mayor’s liberal advocacy group, the Progressive Agenda Committee, also announced plans to hold a forum for presidential contenders in Iowa in December, complicating Mr. de Blasio’s calculus about whether to remain unaffiliated with a candidate.