The first lady told a roundtable of nutritionists hosted by the White House that Congress shouldn't "play politics with our kids’ health." ( via YouTube)

First lady Michelle Obama told a group of school nutrition experts Tuesday that “we have to be willing to fight the hard fight” against Republican proposals on Capitol Hill that would permit a delay in enforcing new school lunch standards.

“The last thing we can afford to do right now is play politics with our kids’ health,” she told a roundtable meeting hosted by the White House. “Now is not the time to roll back everything we have worked for.’’

Her sharply worded foray into a Capitol Hill dispute startled backers of legislation that would allow some districts to temporarily opt out of new federal mandates to increase whole grains, fresh fruits and vegetables in school lunches.

“These new federal regulations should not drive local school nutrition programs under water,’’ said Brian Rell, spokesman for Rep. Robert B. Aderholt (R-Ala.), who sponsored House legislation that would grant qualifying school districts an opportunity to postpone enforcement of the new rules. “This temporary one-year waiver simply provides them a lifeline,” he said, noting that only districts that lost money in part of the past year would qualify for the waiver.

Leaders of an influential lobbying group, the School Nutrition Association (SNA), also expressed surprise at the vehemence of the criticism.

“SNA does not see the waiver as a rollback but as a way to hit the pause button,” said Leah Schmidt, the president of the group, which had supported the new school menu standards when they were approved in 2010 as part of the Healthy Hunger-Free Kids Act. A pause is necessary, Schmidt said, because many schools are overwhelmed by the new requirements and are seeing dramatically increased waste and cost, while sales decline.

In a conference room at the Eisenhower Executive Office Building on Tuesday, the first lady heard a different story as she met with several past presidents of the SNA. They unanimously praised the program and expressed disappointment that the SNA is advocating waivers for a program it once championed.

“This has the potential to do real damage,” said Helen E. Phillips, director of school nutrition for public schools in Norfolk, Va., echoing a widely held view that the waivers could slow or even halt important improvements in student nutrition. Phillips was president of the organization two years ago when SNA supported the new dietary standards. She said she is mystified by the group’s changed position.

Phillips and the eight other nutrition experts meeting with Obama said the new dietary standards are working — and working well.

“Students want it, families want it — and they are participating,” said David Binkle, deputy director of Food Services for the Los Angeles Unified School District, who made the case Tuesday that the more healthful menu is producing positive results. “It is no coincidence that our test scores are up, attendance is up and graduation rates are up,” he said.

Beyond the anecdotal evidence, the first lady cited reports that 90 percent of districts are in compliance with the 2010 law, a figure SNA leaders dispute.

The first lady suggested Tuesday that it might be time to reach out to those nutrition directors who are having trouble meeting the standards.

“Can we shift the conversation to what we need to do to help school districts that are suffering?” she asked. Schmidt, the SNA president, said, “We would love to work with the first lady on this.” But, she added, “we need relief.”