Teachers' union is expected to make its endorsement Wednesday.
The union representing New York City school principals is backing former city comptroller William Thompson Jr. for mayor.
President Ernest Logan of the Council of School Supervisors and Administrators said Tuesday that the union's 16,000 members will do all they can to elect Thompson.
The United Federation of Teachers is expected to make its endorsement on Wednesday.
Both unions have clashed with Mayor Michael Bloomberg over issues including school closings and teacher and principal evaluations. Bloomberg said Monday that the UFT endorsement would be "almost the kiss of death."
Thompson, a Democrat, is the former president of the Board of Education.
Neither the teachers' union nor the principals' union made an endorsement in the 2009 mayoral race. In 2005, the principals' union endorsed the losing Democratic candidate, Fernando Ferrer.