Supporters of Bernie Sanders gathered at a union hall in Seattle for a "Labor for Bernie" rally Sunday. Early in the event, attendees were told that one speaker wouldn't be taking the podium as planned.

Jeff Johnson, the head of the Washington State Labor Council, had been pressured not to speak at the Democratic presidential candidate's event, an emcee told the crowd. A major international labor union had threatened to pull funding from Johnson's group if he did, the emcee said.

Johnson's group is affiliated with the AFL-CIO, the premiere labor federation in the U.S. The AFL-CIO has not yet endorsed anyone for president, and until it does so, state councils like Johnson's are expected to stay in line and not publicly back any particular candidates. Johnson easily could have rankled some unions -- ones backing Hillary Clinton, anyway -- by appearing at a Sanders powwow.

The union that allegedly pressured Johnson went unnamed at the rally. Contacted by The Huffington Post, Johnson politely declined to take any questions on the matter. But labor sources familiar with the situation said the union was the American Federation of State, County and Municipal Employees.