Unions have enjoyed a substantial rise in public support in recent years — but especially last year, when many at-risk workers most needed allies. As workers faced severe illness and financial devastation, organized labor notched its highest approval ratings in nearly two decades.

But now some unions seem keen on frittering away that goodwill by opposing coronavirus vaccination mandates. In so doing, they’re jeopardizing public health, the safety of their members and, ultimately, their own political influence.

The delta variant is sweeping the country, and a large fraction of Americans still refuse to get vaccinated. Because cajoling and even bribing people to protect themselves and their families don’t seem to be working, federal state and local governments have begun requiring that public employees and other public-facing workers get shots, with some reasonable accommodations.

California, for instance, announced this week that state employees and health-care workers would soon have to show proof of vaccination or get tested at least weekly. New York City, while encouraging private employers to require vaccination, announced a similar get-shots-or-be-tested mandate for municipal workers. New York state soon followed. D.C. says its own mandate is in the works for government employees.

The Department of Veterans Affairs announced this week that its health-care personnel must get vaccinated, and President Biden said Thursday that all federal workers and on-site contractors who cannot attest to being fully vaccinated will face strict testing and masking requirements.

To their credit, some unions — such as one representing New York City teachers and another representing federal government engineers — have supported these announcements. These mandates, after all, will promote workplace health and safety, which have historically been among organized labor’s greatest priorities and achievements.

“We don’t want any more of our members dying,” the president of the International Federation of Professional and Technical Engineers said in a statement.

The president of the AFL-CIO, a federation of more than 50 unions, has also eloquently explained why such policies advance both organized labor’s mission, and the economic recovery.

“If you come back in and you’re not vaccinated, everybody in that workplace is jeopardized,” Richard Trumka told C-SPAN. “If we don’t know whether you have been vaccinated or not, we can’t make the proper accommodations to make sure that you are protected and everyone else is protected.” He added that higher vaccination rates can prevent the development of more virus variants, which threaten the economy.

Unfortunately, other unions, including many affiliated with the AFL-CIO, disagree.

In New York City, the president of the union representing paramedics, EMTs and fire inspectors said that only about half his members are vaccinated (versus roughly 71 percent of adults in the city overall who’ve received at least one shot). He said that the mayor’s policy — which, again, exempts workers from vaccinations if they get tested regularly — is a “civil liberty being taken away from us.”

That union has demanded overtime pay for workers who decline vaccines and then get tested outside of normal work hours — which effectively means paying people not to get vaccinated. No way that incentive could backfire.

Other public- and private-sector unions around the country representing firefighters, postal workers, law enforcement officers, teachers and hospital workers have also criticized vaccine mandates, even as some of these same unions say they’re encouraging their members to get inoculated voluntarily.

It’s not entirely clear what’s driving their opposition.

Maybe some unions have been captured by the cranks in their ranks. Maybe some labor leaders are posturing to extract other concessions. The executive director of New York City’s largest municipal union implied as much when he said vaccination and testing policies can be implemented only through bargaining. (“New York City is a union town, and that cannot be ignored,” he said.) The Association of Flight Attendants recently negotiated an extra three days’ vacation for members who (voluntarily) get vaccinated.

Maybe, absent more meaningful deliverables for members (such as bigger raises), some labor leaders are fussing over testing and shots to show members they’re still useful.

Some union officials have suggested that mandates — even with those exemptions and accommodations — will backfire, and cause vaccine-resisters to resist more. If that’s the case, it suggests union officials must show more leadership on this issue, not less. Their job is to protect their workers from real threats, not the fever dreams of conspiracy theorists. That requires looking out for their most vulnerable members, including those who are immunocompromised, or who have at-risk children not yet eligible for vaccination, and who need to know that their workplaces are safe.

Labor leaders advocated admirably last year on behalf of workers whom regulators and companies had failed to protect. Today, public officials and (some) corporations are finally stepping up and mandating measures that can make workplaces safer, and enable the economy to recover faster. If “Big Labor” obstructs this effort, it will fail not only its own members, but also the many admirers and political allies it worked so hard to win over.