It appeared as if Mayor Bill de Blasio would finally get his moment: “Rally for Hillary in Manhattan,” the mayor’s announcement said.

But the event on Wednesday evening was a bit less than it seemed: Hillary Clinton was not there.

About 300 people, mostly union members and employees, attended the event, held in a meeting room at the Lower Manhattan offices of the United Federation of Teachers. It featured the mayor; Mrs. Clinton’s campaign manager, Robby Mook; Representative Joaquin Castro of Texas; Gale A. Brewer, the Manhattan borough president; and the federation’s president, Michael Mulgrew.

High praise was everywhere. Mr. Mook said Mr. de Blasio, a Democrat, was the “greatest mayor in the history of New York.” The mayor praised the toughness of Mrs. Clinton, who spent the day in Pennsylvania.

Despite the exchange of compliments, the event showed what has become obvious in recent months: Mr. de Blasio’s fence-sitting before finally endorsing Mrs. Clinton, rather than Senator Bernie Sanders of Vermont, in the Democratic presidential race has stranded him on the fringe of the action.