AFL-CIO chief Richard Trumka is warning labor leaders to hold off on endorsing Sen. Bernie Sanders’ bid for the presidency, saying the federation’s bylaws specify that such endorsements are to be left up to the organization on a national level.

Trumka, in a memo sent out this week, reminded groups that they are not allowed to “endorse a presidential candidate” or even work on statements or resolutions that indicate a preference for any candidate, reports Politico. Further, he said that “personal statements” are also forbidden.

“Because in years past, and already this year, a number of questions have been raised,” Trumka said, “I want to remind you all that the AFL-CIO endorsement for president and vice president belongs to the national AFL-CIO.

“State federations, central and area labor councils, and all other subordinate bodies must follow the national AFL-CIO endorsement regarding president and vice president.”

Under the organization’s procedures on endorsement, a political committee makes its recommendation to the executive council in Washington, which then submits it for ratification by leaders of its member unions. A two-thirds majority is required to approve the endorsement.

Trumka said the AFL-CIO had sent out questionnaires to both Democrats and Republicans, with a Friday deadline, and plans to interview candidates during its July executive council meeting.

National union leaders, though, are drawn to the party’s more progressive side, represented by Sanders, an independent running for the Democratic nomination, and groups in South Carolina and Sanders’ home state of Vermont have already passed resolutions that support him. Some union leaders in Iowa are also calling for a resolution to be passed at their convention in August to back Sanders.