LOS ANGELES, OCT. 29 -- California's "Big Green" environmental initiative, considered a certain winner even by opponents six months ago, is close to defeat with potentially serious consequences for the U.S. environmental movement.

Although Mervin Field's California Poll once showed the initiative, the most ambitious antipollution measure ever considered by American voters, ahead by more than 2 to 1, Field said last week that it now has no better than a 50-50 chance of approval. A Los Angeles Times Poll released Saturday showed it trailing 44 to 42 percent, with the losing margin as much as 12 percentage points among those most likely to vote.

Conservative critics of the initiative, listed as Proposition 128 on the Nov. 6 ballot, have been joined by the usually liberal Los Angeles Times and Sacramento Bee as well as Assembly Speaker Willie Brown (D). A new television spot, in which former U.S. surgeon general C. Everett Koop says the proposition "would not protect Californians' health," has begun to have an impact.

The initiative's supporters, including nearly every major environmental organization in the country, insist that it will pass. Paul J. Allen, director of communications for the private, nonprofit Natural Resources Defense Council (NRDC), predicted that frustrated voters "are going to say to themselves this is too important to let the politicians muck it up."

California Assemblyman Tom Hayden (D), recently divorced from wealthy actress Jane Fonda, has lent $830,000 of his own money in the last few days to the "Yes on 128" campaign to help finance a last surge of television commercials. He also pledged not to run for the environmental advocate's post that would be created by the proposition, countering critics who said he was trying to create a job for himself. Consumer advocate Ralph Nader held news conferences today in Los Angeles and San Francisco to rally support for the endangered initiative.

"Yes on 128" spokesman Duane Peterson said that although initiative proponents expect opponents to outspend them $16 million to $4 million, "we're not going to lose." The opponents include oil, chemical and timber companies.

But according to several independent analysts, defeat of Prop 128 now appears likely even though opponents once thought their cause so hopeless that they created a rival initiative with weaker pesticide controls -- Proposition 135 -- to try to kill Prop 128 indirectly.

Proposition 128, spelled out in 16,000 words, would ban all pesticides used on food if they include chemicals known to cause cancer in laboratory animals. It also would phase out chemicals that deplete the atmosphere's protective ozone layer, limit emissions of gases adding to the "greenhouse" -- global warming -- effect, protect old-growth redwood forests, prohibit oil drilling within three miles of shore except in national emergencies, and establish a $500 million fund for oil spill prevention and cleanup through a 25-cent-a-barrel fee paid by oil companies. An elective post of environmental advocate would be created to enforce its provisions.

Sheldon Kamieniecki, an associate professor of political science at the University of Southern California who specializes in environmental politics, said the uncertain economy may convince many voters that the state cannot afford such a sweeping program. "Its defeat . . . would send a message to politicians and environmental leaders throughout the country that the public does want a cleaner environment but does not want to pay incredible amounts to get it done," Kamieniecki said.

Defeat of Prop 128 also would give heart to several scientists around the country who have been fighting new federal and state environmental controls that they argue do more harm than good. Koop's anti-Proposition 128 commercial, which even proposition supporters called unusually effective, emphasizes this point. "Public policy should be based on sound science, not scare tactics," Koop says in the 30-second spot. "If this would protect the health of mothers and children, as its proponents say, I'd be with them. I'm not!"

"There has been virtual hysteria about traces of {cancer-causing} chemicals in our environment," said Bernard Davis, professor of microbiology at the Harvard Medical School, who joined 149 other scientists and health care officials in announcing opposition to Prop 128. University of California at Berkeley biochemist Bruce Ames, their leading spokesman, said there is no evidence that the pesticides affected by Prop 128 cause cancer in humans, while the proposition's ban on those pesticides "would reduce the amount of available fresh produce," which is known to reduce cancer risk.

Elizabeth M. Whelan, president of the American Council on Science and Health, noted recent reports casting doubt on the usefulness of laboratory tests in which rats developed cancer after ingesting huge amounts of certain chemicals.

Allen of the NRDC, who was in California to work for Prop 128, said, "I'm really kind of offended" by Ames's and Whelan's arguments. "Life is very precious and we have a tradition in this country that we don't experiment with people, we don't play with people's lives," he said.

Many new opponents say they sympathize with the initiative's goals, but feel it goes too far. Assembly Speaker Brown said he could not support siphoning off tax dollars to fund the environmental advocate's office. The Times and the Bee called the initiative too costly and unnecessary in light of existing environmental controls.

Some environmental groups already appear to be discounting the importance of a Big Green defeat. Gene Karpinski, national lobbying director for the Public Interest Research Groups (PIRGs), said that if Prop 128 proves too politically unwieldy because of its size and complexity, "the recycling, land use and forestry initiatives also on western state ballots will offer better tests of public support for the environment."