LOS ANGELES -- Already under assault from spreading subdivisions and rising water costs, California's gigantic agricultural industry faces a pivotal electoral test in November that could affect the price and quality of half the nation's fruits and vegetables.

Proposition 128, the most stringent antipollution measure ever placed on an American ballot, would impose unprecedented curbs on pesticides here and perhaps mark a severe change in political fortune for what was once the most influential industry in the nation's most populous and productive state.

Political organizers and their scientific allies on both sides have just begun to expose voters to the intense technical disputes over the agricultural impact of Prop. 128, which also offers protections for water, air and forests and is often called the Big Green initiative.

Supporters say it would remove even the smallest threat of cancer-causing substances in foods and help strengthen antipollution forces nationwide. Its opponents say it would cut production of some fruits and vegetables in half, eliminate 100,000 jobs and even increase ground water pollution in some instances.

Otto Doering, professor of agricultural economics at Purdue University, said outside experts see the Prop. 128 fight as part of a half-century struggle here over issues such as water, land use, migrant labor and the environment that "have become even more intensely felt as population pressure builds, as natural resources become constrained, and as people become more environmentally and socially concerned."

The debate has also focused attention on agriculture's shrinking share of the state economy, and the prospect that the growing ranks of suburban grocery shoppers concerned about pesticides may this year give the industry its worst defeat.

"It's $20 billion out of a $600 billion economy, 350,000 jobs out of 14 million jobs," said Stephen Levy, senior economist at the Center for Continuing Study of the California Economy. Reporters have occasionally called agriculture the state's biggest industry, but such a description is based on broad definitions of farming and narrow definitions of other enterprises. Even including indirect impacts, agriculture generates no more than 5 percent of the state's income, Levy said.

That does not change its vital place in the lives of consumers here and elsewhere, for the state produces at least half of the nation's fruits and vegetables. Opponents of Prop. 128 have emphasized what banning pesticides with even tiny amounts of cancer-causing substances might do to prices. A report by San Francisco-based Spectrum Economics Inc. for the state Chamber of Commerce and the California Manufacturers Association predicted 10 to 50 percent reductions in harvests of almonds, grapes, lettuce, oranges and strawberries, with resulting price increases. "Consumers would spend more money to eat foods of lower quality and variety," the report said.

Don Schrack, spokesman for the "No on 128" campaign, said that whatever the size of California agriculture today, "one out of every five jobs is related to it in some way" and "more than 100,000 people are projected to be out of work" if Prop. 128 passes.

Another recent Spectrum report predicted annual revenue losses for California state and local governments of up to $12 billion if the measure is passed.

The report said the initiative's many requirements, including a 20 percent reduction of carbon dioxide emissions by the year 2000 and a ban on cancer-causing substances in pesticides, would cut the state's economic growth by 1 to 2 percent, reducing the economic base that supports local government.

The report said the restrictions on carbon dioxide, one of the "greenhouse gases" suspected of causing dangerous warming of Earth's atmosphere, would also increase state and local government energy and transportation spending by more than $100 million a year. "California taxpayers simply can't afford Proposition 128," said Larry McCarthy, president of the California Taxpayers' Association.

Duane Peterson, spokesman for "Yes on 128," said the initiative's benefits would far outweigh its costs. "Unhealthy air in Southern California alone costs at least $9 billion every year," he said. "Big Green will result in tremendous savings to Californians in reduced health care and insurance costs, fewer lost work days due to illness and less premature deaths due to cancer."

Yet another new report, "Lies, Damned Lies, and Statistics," by the pro-128 Natural Resources Defense Council (NRDC) says the agricultural impact of the initiative would be small and calls the Spectrum Economics analysis "fundamentally flawed," particularly in its "gross exaggeration of the number of pesticides that would be phased out." The NRDC report says Prop. 128 would eliminate 20 pesticides; the Spectrum report for the Chamber of Commerce lists at least 31. Spectrum also says three important ingredients in many pesticides, copper, sulfur and oil, would be banned, an assertion Prop. 128 supporters vehemently deny.

Steven J. Moss, project manager for the Spectrum report, said he has been told the drafters of Prop. 128 "didn't intend" to ban the three substances but "made a mistake." Peterson called this "hogwash" and outlined the drafters' legal interpretation, but the issue pivots on differing definitions of a "contaminant" under the law and will likely go to the courts if the initiative passes.

Both campaigns assume voters -- virtually evenly divided on the question, according to the latest poll by the Los Angeles Times -- have little patience with such details. Their commercials emphasize much broader and more political themes. The pro-128 forces promote the image of a cleaner world and emphasize their opponents' ties to huge chemical and agricultural companies. The opponents label Prop. 128 "the Hayden Initiative" to create the impression that state Assemblyman Tom Hayden (D), the Yes on 128 campaign chair, has fashioned the initiative so he can run for environmental advocate, the new statewide post mandated by the initiative.

At a recent Berkeley symposium on Prop. 128, Doering of Purdue expressed a view rarely heard here -- that decades of debate over California agriculture have made it "the most regulated in the nation with the most stringent and best-enforced safety, health and agricultural labor rules."

David Bunn, the research director of Pesticide Watch who is actively supporting Prop. 128, acknowledged that California is "one of a handful of the best programs in the country," but took little comfort from this.

"I say to that, God help the rest of the country," he said.