In a memo this week to state, central and area divisions of the labor federation, and obtained by POLITICO, the AFL-CIO chief reminded the groups that its bylaws don’t permit them to “endorse a presidential candidate” or “introduce, consider, debate, or pass resolutions or statements that indicate a preference for one candidate over another.” Even “‘personal’ statements” of candidate preference are verboten, Trumka said.

The memo comes amid signs of a growing split between national union leaders — mindful of the fact that Clinton remains the undisputed favorite for the nomination — and local officials and rank and file, who are increasingly drawn to the Democratic Party’s growing progressive wing, for whom Sanders is the latest standard-bearer.

The South Carolina and Vermont AFL-CIOs have passed resolutions supporting Sanders, and some local AFL-CIO leaders in Iowa want to introduce a resolution at their August convention backing the independent senator from Vermont. More than a thousand labor supporters, including several local AFL-CIO-affiliated leaders, have signed on to “Labor for Bernie,” a group calling on national union leaders to give Sanders a shot at an endorsement.