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The largest teacher’s union in the United States is calling on Walmart to stop selling guns.

The American Federation of Teachers also demanded that the company stop donating to politicians who take money from the National Rifle Association and threatened a boycott if the retailer didn’t take action.

“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,” the union’s president, Randi Weingarten, said in a letter addressed to Walmart CEO Doug McMillon.

The union also called on the company to fund gun buyback programs and to lead a national discussion with corporate leaders on making the country safer.

“Walmart has millions of customers, and they should all feel safe while shopping,” the letter reads. “If you choose to act, it could change our national conversation in an instant.”

The American Federation of Teachers represents 1.7 million members, according to its website. Walmart did not immediately return CNN’s request for comment.

The company stopped selling assault rifles after the Sandy Hook shooting in 2015, and raised its minimum gun purchasing age to 21 after last year’s school shooting in Parkland, Florida. It also does not sell handguns outside of Alaska.

But as the nation’s largest retailer, Walmart is still one of the largest sellers of guns and ammunition in the country. Now after a series of recent deadly shootings, the company is facing increased pressure to stop selling guns.

Walmart stores were the site of two shootings in recent weeks. A white nationalist is accused of killing 22 people inside a Walmart store in El Paso, Texas, on Saturday. Two employees were killed in a shooting at a Mississippi Walmart on July 30.

Nine people were also killed Sunday in a shooting in Dayton, Ohio.

In light of those shootings, Walmart said it would pull displays of violent video games from its stores. But so far, it hasn’t announced changes to its policies on displaying and selling guns.

McMillon vowed a “thoughtful and deliberate” response to the shootings in a statement earlier in the week.

“We will work to understand the many important issues that arise from El Paso and Southaven, as well as those that have been raised in the broader national discussion around gun violence,” he said in a memo to employees.