Calls for Walmart to stop selling firearms grew louder as a coalition of unions and organizations said they would stage rallies nationwide this weekend to pressure the country's largest retailer to use its influence to push gun reform.

Guns Down America says that Color of Change, MoveOn, and the American Federation of Teachers are among the organizations joining it at Walmart stores across the U.S. to not only call on the company to stop selling guns, but demand that it support gun buybacks and use its clout to push for a ban on military style weapons.

The coalition also wants its campaign to go viral, launching the #Walmartmustact challenge. Visitors to Walmartmustact.org can print out a letter asking the retailer to stop supporting NRA backed politicians along with the other demands. Then they're being asked to take a picture, tag their friends, and deliver the letters to a Walmart store manager.

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"We really believe that given its enormous size, Walmart will be able to shake up this conversation ... both in Congress, but also with the gun industry and gun manufacturers,'' says Igor Volsky, founder and executive director of the group Guns Down America. "They need to begin using those relationships, using their leverage, using their clout, to do everything they can to keep their customers safe. To keep their employees safe ... I think that's their responsibility.''

A Walmart store in El Paso, Tx was the site of one of two mass shootings the weekend of Aug. 3 that together left 31 people dead, dozens more injured, and the nation tense and shaken. Days earlier, two Walmart managers were killed by a gunman described as a disgruntled employee in a store in Southaven, Mississippi.

In the wake of those shootings, Walmart removed violent video game displays and signs from stores. And last Wednesday, Walmart CEO Doug McMillon said in a note to store employees that "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."

"We will be thoughtful and deliberate in our responses,'' he continued, "and we will act in a way that reflects the best values and ideals of our company, with a focus on serving the needs of our customers, associates and communities."

For now Walmart continues to sell guns. But the pressure to get the retailer to stop is building.

As of Wednesday afternoon, more than 76,000 people had signed a petition on Change.org calling on Walmart to halt firearm sales. Several democratic presidential candidates, including Senators Elizabeth Warren and Bernie Sanders, have echoed that demand.

And last weekend, Guns Down America says 150 people gathered at a Walmart near Marjory Stoneman Douglas High School in Parkland, Fla where 17 people were killed in a mass shooting last year, to deliver a letter asking the location end gun sales.

With legislation that would expand and enhance background checks law languishing in the Senate, advocates say that Walmart can also play a pivotal role in getting conservative lawmakers in particular to take action.

"There is no bigger traditional retailer than Walmart,'' says Fred Guttenberg, whose 14-year-old daughter Jaime was among those killed in Parkland, and whose group Orange Ribbons for Gun Safety is part of the coalition demanding Walmart do more. Walmart "gives the politicians the cover to say 'If Walmart can take such a stand, we need to do so as well.' ’’

The American Federation of Teachers has also joined the effort. In a letter to McMillon dated Aug. 7, Randi Weingarten, the union's president, warned Walmart that if it doesn’t take action, its 1.7 million members might take their business elsewhere.

“If you choose to act, it could change our national conversation in an instant,’’ Weingarten wrote. “And 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.’’

Walmart has taken actions around the sale of weapons and ammunition in the past that have drawn praise from some gun control advocates, including stopping the sale of handguns everywhere but Alaska, ending sales of modern sporting rifles and starting last year, prohibiting sales of firearms to anyone under 21.

But Guttenberg says that's not enough.

"We can’t continue going forward the way things are,'' he says. "We’ve got to make a change and that involves government and legislation and changes of law. But it also involves business ... It is time for them to demonstrate that they're ready to be a part of this cultural change.''