First, the nation agreed that its children are too heavy and unhealthy.

Then, the federal government - Democrats and Republicans together channeling scientific research - hammered out ideas to reduce fat, calories, and salt in school meals.

Now, that harmonious effort is splintering as a food fight embroiling Congress, health professionals, the White House, and even cafeteria workers threatens to rage through the summer and disrupt lunch period come September.

In an odd twist, the School Nutrition Association (SNA) has teamed with House Republicans to try to give schools the chance to opt out of the very nutrition standards the SNA helped bring about. The association represents 55,000 food-service workers, from program directors to the people who prepare and dish out the food.

So surprising is the reversal that 19 past presidents of the SNA united this spring to ask the organization to reconsider its stance.

Puzzled and outraged, health professionals and antihunger advocates say politics and big money may have had a hand in what should have been a simple plan to help children eat better. SNA officials on Monday denied the charge.

Plans to ameliorate school meals came to fruition in 2010 with the passage of the federal Healthy, Hunger-Free Kids Act, which had bipartisan support. The Institute of Medicine, the health arm of the National Academy of Sciences, assessed school meals and came up with healthier alternatives.

The SNA was a major backer of the act, as were about 200 health and nutrition nonprofits nationwide, plus President Obama and his wife, Michelle.

Since 2012, when the new nutrition standards took effect, more than 90 percent of U.S. schools have complied, according to the U.S. Department of Agriculture, which administers the school meals program. Under the new rules, fat has to be kept under 35 percent of calories, and trans fat has to be eliminated. Saturated fat must be under 10 percent; every meal has to include fruits and vegetables; grains must be whole; and sodium has to be lowered.

The results have started to register. The 32 million children who eat school meals everyday have been eating 16 percent more vegetables and 23 percent more fruit since the implementation of new rules, according to Sue Nelson, vice president of federal advocacy for the American Heart Association.

"We think the program is working," she said. "It's been a huge win for kids' health."

Other health professionals reported seeing child obesity rates leveling off.

But in 2011, the SNA appeared to reverse itself, saying the new regulations were too expensive for school districts to maintain. Since whole-grain, low-sodium foods often cost more than the usual fare, the organization asked that schools be given the chance to opt out of nutritional guidelines, at least for a year.

Late last month, the House Appropriations Committee passed a bill that could make that happen if Congress agrees.

Both moves have sparked outrage.

Health and antihunger advocates said that at the same time the House had allowed partisan politics to interfere with children's health, the SNA had succumbed to food-industry pressure to keep the status quo.

Margo Wootan, director of nutrition policy for the Center for Science in the Public Interest, said the SNA is "getting bad advice" from a Republican-leaning lobbying firm that is looking to weaken any regulation supported by the White House in an election year.

She added that she was "stunned to learn" that much of SNA's funding comes from food industry companies.

"Their industry members have significant influence," she said.

Colin Schwartz, government affairs manager for the Physicians Committee for Responsible Medicine, agreed. The committee is a Washington-based nonprofit that promotes preventive medicine and advocates for plant-based diets as a way to prevent disease.

"It is suspicious that SNA has gone from being supportive of the child nutrition measure to being against it," he said. "Food companies are supportive of the waiver for schools to opt out because they could potentially lose money on their unhealthy products."

An SNA spokeswoman had no comment about its new lobbying firm.

Leah Schmidt, president of the SNA, issued a statement denying that her group was influenced by food companies.

"These speculations are false, unsupported by credible sources, and serve no purpose other than to overly politicize the debate and detract from the legitimate concerns of school nutrition professionals nationwide," Schmidt said.

Those concerns are real, said Brian Rell, chief of staff of U.S. Rep. Robert B. Aderholt (R., Ala.), chairman of the House Appropriations agriculture subcommittee.

He said lunch participation is going down, and the higher expense of healthy foods threatens the existence of some school meal programs.

Karen Castaneda, director of nutritional services for the Lower Merion School District, said it's too hard to adhere to the ambitious new health standards.

"The intention of the new program is good, but it's too much too fast," she said.

While it is difficult to comply with the guidelines, Philadelphia schools will continue to do so, said Wayne Grasela, director of food services for the School District. "It's costly for us, but we're very much in favor of the changes," he said.

Describing himself as frustrated with the stances of both the SNA and the House, Agriculture Secretary Tom Vilsack said in an interview that before asking for a waiver from nutrition guidelines, school districts would have to open their books to the Department of Agriculture to show how serving healthier food would cost them money.

And, he added, they would do so at their peril.

"I don't think school districts want me to ask to see their books," Vilsack said.

Vilsack said that if a district were shaving money off the food program to support athletics, for example, "I'd have to be convinced it was [the nutrition program] and not something else that was stressing the budget."

He added: "It's just not the right thing to do to take a step backward and opt out of the program."