Last week, the organization released a study it had commissioned that estimated the initiative would add $1.2 billion in costs for California farmers and food producers. Ms. Fairbanks said that the higher costs could add as much as $350 to $400 to an average family’s grocery bill.

In addition, she said, the opponents believe the labeling would heighten what they call unfounded concerns about the safety of genetically engineered crops.

The European Union has required such biotech labeling since 1997, and companies by and large have formulated their products so that they do not contain any genetically engineered ingredients and thus do not need labeling. Also, David Byrne, the former European commissioner for health and consumer protection, has said that there was no impact on the cost of products.

But for more than a decade in the United States, most processed foods like cereals, snack foods and salad dressings have contained ingredients from plants whose DNA was manipulated in a laboratory. Regulators and many scientists say they pose no danger.

Americans, however, are becoming much more aware of the role that food plays in their health and well-being, and consequently want much more information about what they eat, including whether it contains genetically engineered ingredients as well as salt and trans fats. So far, opponents of Proposition 37 have committed roughly $25 million to defeat it, with the largest contributions coming from Monsanto ($4.2 million) and DuPont ($4 million), which have made big investments in genetically engineered crops.