Teachers spend a lot of money out of pocket on school supplies, so when a national teachers union threatens to boycott a retailer that sells school supplies, it’s a threat with teeth. The American Federation of Teachers is threatening to boycott Walmart over the chain’s gun sales and support for politicians blocking gun law reform.

“If Walmart continues to provide funding to lawmakers who are standing in the way of gun reform, teachers and students should reconsider doing their back-to-school shopping at your stores,” AFT president Randi Weingarten wrote to Walmart CEO Doug McMillon.

It’s not clear how or when the AFT will move forward, but the union does have a history of taking action to pressure companies on gun issues. CBS reports that “Last year, the AFT dropped a preferred mortgage lender program with Wells Fargo after the giant bank declined to cut ties to the NRA.”

Sign the petition to Walmart: Stop selling guns and ammunition.