But the endorsement of Mr. Sanders also proved divisive within the Working Families Party leadership, which includes a number of influential union leaders who support Mrs. Clinton. Mike McGuire, the political director of the Mason Tenders District Council of Greater New York and a Clinton supporter, said in an interview that he believed party officials had “rigged the system to make sure it went for Bernie.”
The party is also closely associated with top New York Democrats, including Mayor Bill de Blasio, who have already given their support to Mrs. Clinton’s campaign. A spokesman for Mr. de Blasio said the mayor had no role in the Working Families Party’s endorsement.