Whether the measure, an amendment to the farm bill, can survive the convoluted politics that have bogged down that legislation in the Senate is one issue. Whether it can survive the battle among factions in the fight to improve school food is another.

Senator Tom Harkin, Democrat of Iowa and the chairman of the Agriculture Committee, has twice introduced bills to deal with foods other than the standard school lunch, which is regulated by Department of Agriculture.

Several lawmakers and advocates for changes in school food believe that an amendment to the $286 billion farm bill is the best chance to get control of the mountain of high-calorie snacks and sodas available to schoolchildren. Even if the farm bill does not pass, Mr. Harkin and Senator Lisa Murkowski, Republican of Alaska, a sponsor of the amendment, vow to keep reintroducing it in other forms until it sticks.

They are optimistic about their chances because there is more public interest than ever in improving school food and because leaders in the food and beverage industry have had a hand in creating the new standards.

But that intense corporate involvement, along with exemptions that would allow sales of chocolate milk, sports drinks and diet soda, has caused a rift among food activists who usually find themselves on the same side of school food battles.

“This pits ideals about what children should eat at school against the political reality of large food corporations insisting their foods be available to children at all times,” said Marion Nestle, a professor at New York University and the author of two recent books on food politics and diet. “The activists want vending machines out of schools completely.” Dr. Nestle has taken no public stand on the measure.

The nutrition standards would allow only plain bottled water and eight-ounce servings of fruit juice or plain or flavored low-fat milk with up to 170 calories to be sold in elementary and middle schools. High school students could also buy diet soda or, in places like school gyms, sports drinks. Other drinks with as many as 66 calories per eight ounces could be sold in high schools, but that threshold would drop to 25 calories per eight-ounce serving in five years.

Food for sale would have to be limited in saturated and trans fat and have less than 35 percent sugar. Sodium would be limited, and snacks must have no more than 180 calories per serving for middle and elementary schools and 200 calories for high schools.

The standards would not affect occasional fund-raising projects, like Girl Scout cookie sales.

Although states would not be able to pass stronger restrictions, individual school districts could.

The rules have the support of food and drink manufacturers, including the American Beverage Association, which worked closely on the amendment with Mr. Harkin’s office and the Center for Science in the Public Interest, an advocacy group that has been critical of the food industry.

“This whole effort has momentum because of the variety of interests that have come together who do not usually find agreement,” said Susan Neely, president of the beverage association.

Some parents and nutritionists are angry that states will not be able to enact even tougher limits.

“My little fights in school districts are just going to be harder and harder because they’ll say, Well, here are the federal guidelines,” said Dr. Susan Rubin of Chappaqua, N.Y., a nutritionist who helped found the Better School Food advocacy group.

“It’s crazy to think we are going to fix children’s health just by letting companies sell schoolchildren smaller portions of Gatorade and baked chips,” she said.

Margo Wootan, director of nutrition policy at the Center for Science in the Public Interest, has long been a critic of companies that produce food that she considers unhealthy and of government policy toward them.

That is why some of the center’s allies were surprised that Ms. Wootan had worked so closely with manufacturers on the standards. Conversely, she was surprised to find herself on the defensive for finally arranging food limits that actually have a good chance at becoming law.

“I do not understand why some groups would try to stand in the way of legislation that is going to get soda, snack cakes and other high-fat, high-salt food out of virtually all schools,” she said.

Correction: December 3, 2007

Because of an editing error, an article yesterday about a proposal to limit the types of food and beverages for sale at schools was imprecise in some editions about the extent of the proposal. It covers food served on special à la carte lines, in vending machines and in snack bars. It would not cover standard school cafeteria food, which must meet Department of Agriculture nutritional standards.