The poll showed broad support among voter groups, but the interviews took place before Tuesday's start of a major television advertising blitz by opponents aimed at changing voters' minds on the issue.
So far, the opposition campaign has raised more than $32.5 million, collected mostly from businesses affected by the measure.
The first 30-second television spot complains that passage of the labeling initiative would foster more government bureaucracy and send food prices spiraling. The commercial features Central Valley farmer Ted Sheeley, who grows corn, cotton, tomatoes, pistachios and other crops. He warns that "the people least able to pay are going to be forced to pay more" for food. It calls the measure "the deceptive food labeling initiative."
Proposition 37 is sponsored by a coalition of farmers, food makers, retailers and consumer groups, mainly from the organic movement that touts a message that shoppers in California have "a right to know" what's in the food they eat.
If approved by voters Nov. 6, the labeling initiative would make California the first state in the nation to require labels on genetically engineered crops or processed foods, such as corn, soybeans, sugar beets and Hawaiian papayas. It would require labels on supermarket shelves or on food packages.
The California ballot issue is being watched closely by experts who say it could set the stage for battles in other states and perhaps thrust the issue of labeling genetically modified organisms to the forefront in Washington.
The telephone poll of 1,504 registered voters statewide was conducted Sept. 17 to 23. It showed majority support among most age groups, geographic areas, ethnic groups and educational levels.
The proposition has generated plenty of strongly held opinions that were reflected in interviews with poll participants.
"I think everyone should probably know what they're buying and what's in it," said Dena Gunnoe, a gym co-owner in La Mesa near San Diego and a registered Republican. "If they don't care, (the labeling) doesn't harm them, and, if they do care, they have the option to look on the package and see how things are made."
But another Republican, Jane MacNeil of Arcadia, said Proposition 37 would hit low-income people with higher food prices. "People who go to Trader Joe's care about labeling," she said. "People who go to the 99 Cent Store don't care as much."
The new poll results are heartening to supporters.
"There's overwhelming support," said Proposition 37 spokeswoman Stacy Malkan, "across all demographics and age groups, and that's pretty consistent with national polls that have shown support for GMO labeling."
Labeling coalition members contend that the health effects of GMO foods are unknown and that more scientific studies are needed to determine whether they are safe.
As of Sunday, the Yes on 37 campaign had raised $3.5 million, with its largest contribution of $1.1 million coming from Mercola.com Health Resources, a privately held Illinois company that operates a "natural health" website.
Opponents — primarily biotech companies and some of the best known manufacturers of packaged foods, including Nestle, Coca-Cola and Kellogg, say genetically modified foods are safe. They denounce Proposition 37's "scare" tactics and stress that the U.S. Food and Drug Administration says there is no difference between modified and non-modified plants.
The No on 37 campaign's largest donor as of Sunday was Monsanto Co. with $7.1 million, according to Maplight.org, a nonpartisan campaign finance tracking service. Monsanto markets seeds to grow corn, soybeans and other crops genetically modified to resist herbicides and pesticides.
Opponents of Proposition 37 face a big challenge in changing voters' minds, said Dan Schnur, director of the Jesse M. Unruh Institute of Politics at USC. "Something called genetically modified food sounds really scary, so it's not surprising that the support for (Proposition 37) is this large," he said.
"It's going to take a tremendously well-funded opposition campaign to get people to move past a visceral reaction against what that type of language represents to them."
The poll was conducted by Greenberg Quinlan Rosner Research, in conjunction with American Viewpoint, on behalf of the USC Dornsife College of Letters, Arts and Sciences and the Los Angeles Times. The margin of sampling error was 2.9 percentage points.
Pollsters from both firms agreed that Proposition 37 is ahead — for now — but faces challenges as election day nears and the opposition campaign unfolds.
"It looks like it's going to pass," said Stanley B. Greenberg, chief executive of the Greenberg firm, a Democratic-aligned pollster. But David Kanevsky, research director at the Republican-oriented American Viewpoint, was more cautious. The results bode well for the measure — "if there was no campaign against it," he said.
Some of the poll findings appear to indicate that voters might turn against Proposition 37 if they get enough critical information. Support for the initiative dropped to 56% from 61% and opposition rose to 32% from 25% when people were read more than a brief description of the measure and, instead, heard ballot language and a statement of the cost to government of enforcing the proposed law.
"Proposition 37 isn't as simple as it seems," said No on 37 campaign spokeswoman Kathy Fairbanks. "As voters look more carefully at the details, we think they will reject it."