A major anti-Walmart union is staying tight-lipped about billionaire Walmart heiress Alice Walton's six-figure donation to Hillary Clinton.

The United Food and Commercial Workers Union endorsed Clinton in January, less than one month after Walton contributed more than $353,000 to a pro-Hillary fund.

International president of the UFCW Marc Perrone wrote in a statement, "It is our members’ strong belief that Hillary Clinton is not just the strongest presidential candidate, but the right one."

The union has sparred for decades with Walmart, going so far as to create the ‘Making Change at Walmart' union group, which represents Walmart workers and frequently tangles with the company in the courts and federal labor system. The website ‘Walmart 1 Percent,' a project of ‘Making Change at Walmart,' features Alice Walton on its "family" page, which draws attention to the wealth of the Walton family.

"The Waltons make billions a year off of a company most of them don’t even work for," the group writes, "while Walmart associates struggle for respect on the job and enough pay to make ends meet."

Clinton served as the only female member of the Walmart Board of Directors from 1986 to 1992, and was at Walmart during a major campaign by the world's largest retailer to defeat labor unions representing its employees.

Good Morning America reported in 2008 that the organization had obtained tapes of Walmart board meetings in which Hillary Clinton remained visibly silent during at least four meetings and "never once rose to defend the role of American labor unions."

John Tate, who served on the board with Clinton, told the New York Times in 2007 that while Hillary was vocal about issues relating to women, she "was not an outspoken person on labor," and that speaking out about labor had the potential to weaken her position on the board. Her time on the board occurred during her husband's service as governor of Arkansas, where Walmart is headquartered.

The Democratic frontrunner has spent years trying to distance herself from her ties to the big box store. Clinton's website, which provides a detailed list of her accomplishments and stances—including her sponsorship of pro-union legislation—does not mention her time on the Board of Directors at Walmart.

"As first lady of Arkansas," her biography states. "Hillary was a forceful champion for improving educational standards and health care access." Though she has received thousands of dollars from various Walmart executives and political action committees, she has—after the donations received media coverage—returned a portion of those donations over the years.

Since Alice Walton's contribution to the Hillary Victory Fund became public, there has been no indication that the donation will be returned and Clinton has not directly commented on the gift. Alice Walton is the daughter of the late Sam Walton, who created the first Walmart in 1962 and spent decades growing the retail company to its current status as the largest in the world.

When pressed about Clinton's time on the board of directors, "Making Change at Walmart" referred the Washington Free Beacon to the UFCW's endorsement statement, which states that the group's "extensive endorsement process found that our members were more concerned about Clinton's plans for the future than her past," noting that "she resigned from any position with Walmart involvement 24 years ago."

The union did not respond to questions about whether knowledge of Walton's contribution would have affected the endorsement.

The Clinton campaign and the Sanders campaign did not return request for comment.