The vote Thursday will require a two-thirds majority of the membership to issue an endorsement. A majority of the 57 member unions are already confirmed Clinton supporters, and the outcome is not in doubt.
The National Nurses Union, which has been very active for Sanders, was among a small number of "nay" votes Friday when the union group's political committee held a voice vote about moving ahead toward a nomination, the union official said. The political committee approved the larger vote to take place next week.
The AFL-CIO is among the biggest endorsement prizes, and one that both Clinton and Sanders had sought. The AFL-CIO typically brings organizing muscle and political money for advertisement and other activities, although it is not yet clear what the organization will do for Clinton.
Each candidate has the backing of some of the individual unions that make up the labor coalition, and Sanders had also captured support from some rank-and-file members of unions whose leadership backed Clinton.
The AFL-CIO declined to issue an endorsement earlier in the primary, although member unions did so.