First lady Michelle Obama has lunch with schoolchildren at Parklawn Elementary School in Alexandria in January. Celebrity cook Rachael Ray is at left. (Pablo Martinez Monsivais/AP)

Sounding a new aggressive tone, first lady Michelle Obama vowed in a private conference call Monday to fight industry efforts at rolling back healthy school-lunch standards, an issue that could come up for a vote on Capitol Hill this week.

The remarks to health activists were made at the beginning of a week of intense lobbying around changes in the national school-lunch program, which sets standards for fat, sugar and sodium levels in food.

“I was thrilled that the first lady pulled advocates together this morning and sounded such a strong rallying cry to fight back against efforts to weaken the school food standards,” said Margo Wootan, who lobbies on Capitol Hill and elsewhere on behalf of the Center for Science in the Public Interest.

Although the phone call was meant to be off the record, several participants were so struck by the first lady’s forceful presentation that they discussed it with a reporter.

A House bill up for consideration this week by the Appropriations Committee would allow schools to apply for waivers from the federally mandated standards if the school’s food program has recorded a financial loss for six months in a row. The Senate Appropriations Committee may take up a similar proposal Thursday.

The first lady has made child health, including nutrition and exercise, a priority since her husband took office in 2009. But her “Let’s Move” campaign has come under criticism from lawmakers and health activists, who say she has shifted from an emphasis on food policy to exercise. They have suggested this was done at the urging of the food industry. The criticism was reflected in a recently released documentary on the food industry, “Fed Up.”

In the conference call, Mrs. Obama urged health activists to fight agribusiness’s lobbying efforts to allow schools to opt out of the mandates to reduce sodium, increase whole grains, and increase servings of fresh fruits and vegetables in lunches.

Also Monday, more than 100 national, state and local nutrition and child advocacy organizations — including the American Pediatric Society, Children’s Defense Fund and Healthy Schools Campaign — circulated a letter asking lawmakers to fight the waiver request.

The waiver proposal is being pushed, in part, by the School Nutrition Association, which represents school nutrition workers and receives substantial industry funding. The group has been criticized by health activists, who say it is serving the interests of processed-food firms such as makers of frozen pizza and agribusiness companies listed as donors.

“Nothing could be further from the truth,” said Julia Bauscher, who is president-elect of the association and directs school nutrition services for the Jefferson County public school system in Louisville. “We are advocating for our 50,000 members who work in school lunch programs and who are struggling to meet the federal requirements.” While the group is generally sympathetic to the goals, she said, the organization’s members have developed “serious concerns about the program, its increased cost and increased food waste.”

Tom Stenzel, president of the United Fresh Produce Association, which is lobbying against delays in carrying out the new rules, said he believes the school nutritionists organization is responding to its donor base.

“They are overwhelmingly dependent on the processed food industry so there is an ingrained bias toward foods like frozen pizza and chicken nuggets that have been a staple of school lunches in the past,” Stenzel said.

Sen. Tom Harkin (D-Iowa), an author of the school lunch standards, rejected the proposal for waivers or other delays.

“Now is the time to hold true to these nutrition standards — as science-based organizations from around the country are asking Congress to do — not undermine public health nutrition programs that have served our children well,” Harkin said in a written statement.

In a separate statement, Agriculture Secretary Tom Vilsack said the waiver proposal would a “major step backwards for the health of American children.” The proposal is for fiscal year 2015. The law that established the standards is up for reauthorization next year.

The standards have come in for criticism from industry, from some schoolchildren and from some GOP members of Congress.

“I want to do all I can to fight childhood obesity,” Rep. Rodney Davis (R-Ill.), who has been sharply critical of the standards, said in an interview. “But I’ve heard from school districts, superintendents, and they are asking for flexibility. This top-down approach from Washington isn’t working. The plate waste is piling up.”

The healthy school lunch program was proposed during the presidency of George W. Bush. Many of the current standards were mandated in the Healthy Hunger-Free Kids Act, passed in 2010 with White House support.

In 2011, Congress adopted special rules recommended by industry that permitted pizza — thanks to its use of tomato sauce — to be counted as a vegetable to meet the law’s requirements. Congress also loosened planned restrictions on the number of times a week that french fries could be served.

Health activists who were on the conference call Monday said she should be lauded for the shift in tactics and tone.

“She has no authority, just charisma and leadership,” said Marion Nestle, a professor of public health at New York University and writer of a blog called Food Politics.

With the Appropriations Committee vote set for later in the week, Wootan said the first lady’s strong views provided a welcome boost.

“I think this will help strengthen the resolve of Democrats in Congress,” she said. “It will remind them how important this program is to children — and to the administration. I do worry also that it may strengthen the resolve of the opposition.”

Alice Crites and Matea Gold contributed to this report.