The nation's second-largest teachers union said it would support its members if they strike in states that reopen schools without safety measures in place.
On Tuesday, the American Federation of Teachers said it would support its 1.7 million members if they choose to strike, but that it should be a "last resort."
The union's president said it "will fight on all fronts for the safety of our students and their educators."
The union wants to wait to fully reopen schools until coronavirus transmission rates in a community fall below 1 percent and daily test positivity rates stay below 5 percent on average — numbers the New York Times says few large school districts have met.
It also wants certain guidelines in place like contact tracing, mask requirements for students and teachers and updated ventilation systems in school buildings.
President Donald Trump wants schools to reopen in the fall so parents can return to work and boost the economy, but education leaders have said hundreds of billions of dollars are needed to reopen safely.