These are bad days for Big Potato.

Okay, you’re right. Nobody calls it Big Potato. It is just five people in a downtown Washington office with potato posters on the walls, a few Mr. Potato Heads scattered around and a modest lobbying budget.

But the National Potato Council has been pushing for years to get equity for the humble white potato in a key federal food-assistance program. And when the revisions to the program were published in the Federal Register this past Tuesday, it was on the losing side — again.

At issue is the Special Supplemental Nutrition Program for Women, Infants and Children, known as WIC, which provides monthly vouchers to low-income pregnant women, mothers and children, and specifies which foods can be bought with the funds.

The new rule made several changes, including adding more whole grains and allowing yogurt as a partial substitute for milk. It also increased the amount allocated for fresh fruit and vegetables to $10 for women and $8 for children. But it left unchanged a provision that has been in place since 2009 that explicitly excludes white potatoes — and only white potatoes — from the list of permissible fresh produce.

The Agriculture Department explained that it was following the recommendations of the Institute of Medicine, adding that “consumption of starchy vegetables meets or exceeds recommended amounts” and that white potatoes are the most widely used vegetable. Including them “would not support the goal of expanding the types and varieties of fruits and vegetables available to program participants,” the USDA said.

For the tuber-curious, the rule makes clear that “orange yams and sweet potatoes are allowed.” But other assortments — such as yellows, purples, reds and fingerlings — fall under the white-potato ban. And for a group that calls itself the “unified voice for the U.S. potato industry,” it was quite a setback.

“We have a problem when USDA tells a WIC participant that potatoes aren’t good enough to be purchased using a WIC voucher,” said Mark Szymanski of the potato council, adding that the group is less concerned with the economic impact of the rule than with how it might damage perceptions of its product.

The potato council, which reported spending $180,000 on lobbying last year, argued that the USDA relied on outdated research and ignored more recent findings that women and children are not consuming enough starchy vegetables.

The group and its members pushed on several fronts, weighing in with the USDA and urging lawmakers to do the same. In July 2012, 93 members of Congress signed a letter to the agriculture secretary asking him to reverse the potato ban. The potato industry was still pushing in late February, when some of the growers who were in Washington for the group’s annual fly-in met with USDA officials and lawmakers to make their case.

The group also urged Congress to act, trying late last year to add language to the farm bill and the agriculture appropriations bill.

Its friends on the Hill predictably included Rep. Mike Simpson (R-Idaho) and Sen. Susan Collins (R-Maine), who spoke a few years ago about one of her first jobs, picking potatoes for an Aroostook County farmer. And the group worked to expand its network beyond the potato-growing North to include members of Congress from urban areas, such as Rep. Gregory W. Meeks (D-N.Y.), who said WIC participants in his diverse district “should be given the opportunity to choose how to feed their families.”

There were a few victories in recent years, including a 2012 prohibition on limiting what vegetables are served in school lunches. A House version of the farm bill that became law this year would have required the USDA to study “the economic and public health benefits of white potatoes on low-income families who are determined to be at nutritional risk,” but that provision didn’t survive the conference committee.

The potato pushers faced some stiff opposition. A group of food, health, science and other advocates, including the National WIC Association, warned that Congress shouldn’t interfere with “sound scientific judgment.” Rep. George Miller (D-Calif.) has been an outspoken critic, arguing that the addition of white potatoes would “displace other more nutritious vegetables” and that potatoes have been associated with health risks such as weight gain.

The potato council’s Szymanski has heard it all before.

“Any time we talk about potatoes and nutrition, the first thing that comes to people’s minds in the nutrition community is french fries,” he said. “We talk about fresh potatoes, fresh potatoes, fresh potatoes, and they hear french fries, french fries, french fries.”

His group’s next move isn’t clear.

Simpson, a member of the Appropriations Committee, inserted a provision in the $1.1 trillion omnibus spending bill signed into law in January that directs the USDA to allow all types of vegetables in the WIC program. If it doesn’t comply, Agriculture Secretary Tom Vilsack has to send a report to Congress within 15 days explaining his actions.

As Szymanski put it, “this isn’t over for us.”