Take the Washington Federation of State Employees, which represents 19,000 people. Although its parent union, the powerful American Federation of State, County, and Municipal Employees, endorsed Clinton in October, the state council thought differently. In January, its executive committee voted to support Sanders and suggested their national leaders take another look at the endorsement process.

“Maybe we need to blow this up,” WFSE public affairs director Tim Welch said. “Yes, we’re a representative democracy here [in the union]—but what if we had all the members directly select who they’d like to recommend? This is an opportunity to rethink how we do endorsements.”

Such talk was not common a year ago. Surveying the presidential race last July, unions looked right and saw a Republican field filled with the likes of Scott Walker, then looked left and saw Clinton flanked by a bunch of random men. Though the American Federation of Teachers was the first major union to endorse Clinton, it was joined within months by heavyweights like the Service Employees International Union and the United Food and Commercial Workers, which represent about 2 million workers each.

This was great for Clinton. An endorsement from SEIU? That’s something every Democrat wants, nearly as badly as Republicans want an “A” rating from the National Rifle Association. What’s more, several of these major labor unions supported Barack Obama during the 2008 primary, lending credibility to Clinton.

The notable exception, however, was the Communication Workers of America. At 700,000 members, it is a sizeable union that primarily represents telecommunications employees. Last December, after a general membership vote, it endorsed Sanders.

That CWA vote marked a break in how things are done. As the Sanders campaign heated up, this led to some hard feelings in other unions. After the American Federation of Government Employees executive board voted to endorse Hillary Clinton in December, an irate worker wrote an angry column in response, titled “Why Did My Union Give an Early Endorsement to Hillary Clinton Over Bernie Sanders?”

Within the first hours of the AFGE's release, the union’s Facebook page was filled with statements in support of Sanders. Within days of the news, the Facebook page “AFGE for Bernie” nearly tripled its membership … Why did AFGE's leadership feel this urgency to endorse so quickly after so many members made it clear they wanted to endorse Sanders or wait to endorse?

That question has preoccupied Larry Cohen. The energetic former president of the CWA is now stumping for Sanders; he spoke with me from a train after joining the senator at a rally with striking Verizon workers. He knows firsthand how many unions make their endorsements from the top down, with limited member involvement, and how much that’s angered people. It means he occasionally has to step lightly.