In recent years, leadership of some of the nation’s largest unions have publicly opposed single-payer health care proposals, angering their rank and file and forcing Democratic politicians who back single-payer to take on a key constituency.

In New York City, for example, the umbrella organization for the city’s public-sector unions — the Municipal Labor Committee (MLC) — recently helped the health insurance industry block a statewide single-payer bill, on the grounds that their members wanted to keep the health care benefits for which they had sacrificed wage increases.

But it turns out that the MLC, which bargains for health care benefits for city unions, was also engaging in backdoor negotiations with the city, resulting in a proposal to switch nearly a quarter million people from Medicare to privately administered Medicare Advantage plans.