Retail giant Walmart is running a back-to-school promotion this summer, encouraging customers to nominate their favorite teachers to win school supplies and a $490 gift card — the estimated amount public school teachers spend out of pocket each year on their classrooms.

On Friday morning, members of the Washington Teachers’ Union slammed the competition as “deceitful” and “bogus.” They argued that the Walton Family Foundation, the charitable organization started by Walmart’s owners, has pumped hundreds of millions of dollars into backing charter schools, which they say are undermining traditional public schools.

“It’s a cynical coverup,” WTUP President Elizabeth Davis said at a news conference outside Moten Elementary School in Southeast Washington.

The union’s criticism of the Arkansas-based retailer stems from its concerns with the Walton foundation, one of the country’s biggest financial supporters of charter schools. The foundation says it has poured $1.3 billion into K-12 education over the past two decades, and it announced in January a commitment of $1 billion to help expand charter schools and other school-choice options nationwide.

[Walton family steps up support for school choice with $1 billion pledge]

Critics of charter schools say the foundation is fueling the privatization of public schools. The teachers union represents teachers in D.C. Public Schools.

Davis said the robust charter sector in the District — nearly half the city’s students attend charters — is stripping taxpayer dollars from traditional public schools. She and other teachers argued that if charters weren’t so influential in the District, perhaps the city’s traditional public schools would have enough money to pay for all the supplies classrooms need.

“Walmart is privatizing all sectors, and we need to put a stop to it,” Davis said.

Walmart and the Walton Family Foundation did not respond to requests for comment Friday.

The District’s charters — public schools that are independently operated — have long said they are serving D.C. students while giving parents more options. Charter supporters bristled at the allegation that they are shortchanging D.C. Public Schools.

“All of our schools are funded on a per-pupil basis,” said Irene Holtzman, executive director of Friends of Choice in Urban Schools, a D.C. charter-school advocacy organization. “Charters are not taking away money from DCPS.”

And DCPS pushed back against the idea that teachers don’t have enough resources.

“DCPS has increased our investments in schools every year for the last five years to ensure that our schools and classrooms have the materials needed for high-quality teaching and learning,” spokeswoman Michelle Lerner said. “We also provide every teacher with $200 to supply their individual classrooms with school supplies.”

[District leaders furious Walmart breaking promise to build stores in poor neighborhoods]

Walmart opened two stores in Northwest Washington in 2013 and another store last year in Northeast Washington. In January, the retailer abruptly announced that it would pull out of two supercenters planned for east of the Anacostia River.