The primary targets, Muller said, would be the giant food makers, but retail grocers also might find themselves in the crosshairs.
"It's easier to win," Muller said. "It's easier to convince the mom-and-pop stores to settle than to convince Monsanto."
That worries grocers such as Ray Martinez, owner of La Playa Market, which caters to lower-income families. For every unmarked item at his Inglewood store, Martinez would need to get a sworn statement from suppliers or get independent certification confirming products are GMO-free.
"It makes no sense to me as a businessman and as a consumer," he said.
Sandler, of the California Right to Know group, said supermarkets would be responsible only for labeling their shelves and would not be penalized for mistakes by manufacturers.
Marsha Cohen, who teaches law at UC Hastings, said the initiative's language on enforcement is very broad and allows anyone to sue for violations. "I would not have chosen to write it in this way," she said.
Cohen, who has no role in either campaign, noted this is a common problem with ballot initiatives, which are not subjected to the same scrutiny as bills passed by the Legislature. There's always some uncertainty about how initiatives that become law should be interpreted, she said.
Cohen would have preferred that Proposition 37 contain a clause spelling out fines for violations that would go to the state's coffers. She also would have limited consumer action to people who have suffered harm from eating mislabeled foods.
San Francisco Bay Area environmental attorney James Wheaton filed the labeling initiative language with the state attorney general's office last year. He also drafted Proposition 65, which requires businesses to post warnings about chemicals, and his firm has won judgments from litigating claims under that initiative, which was passed by voters in 1986.
A message left with Wheaton seeking comment was not returned.
Alicia Chang can be followed at http://twitter.com/SciWriAlicia