* Proportion of likely "Yes" voters drops, "No" gains
* Opposition fundraising leads by margin of over 6-to-1
Oct 11 (Reuters) - An intense advertising blitz, funded by Monsanto Co and others, has eroded support for a California ballot proposal that would require U.S. food makers to disclose when their products contain genetically modified organisms.
If California voters approve the measure in November, it would be the first time U.S. food makers would have to label products that contain GMOs, or ingredients whose DNA has been manipulated by scientists.
For more than a week, the opposition group - funded by Monsanto and PepsiCo Inc - has dominated television and radio air time with ads portraying the labeling proposal as an arbitrary set of new rules that will spawn frivolous lawsuits and boost food prices, positions disputed by supporters.
Experts say the real risk is that food companies may be more likely to stop using GMOs, than to label them.
That could disrupt U.S. food production because ingredients like GM corn, soybeans and canola have for years been staples in virtually every type of packaged food, from soup and tofu to breakfast cereals and chips.
Support for the GMO labeling proposal has plummeted to 48.3 percent from 66.9 percent two weeks ago, according to an online survey of likely California voters conducted by the California Business Roundtable and Pepperdine University's School of Public Policy.
At the same time, the proportion of respondents likely to vote "no" on the measure - known as Proposition 37 - jumped to 40.2 percent from 22.3 percent two weeks ago, according to the survey results released on Thursday.
"Clearly the 'No' side has more money and the advertising is having an effect," Michael Shires, a Pepperdine professor who oversees the survey cited above, told Reuters.
Funding for the effort to defeat the "Right to Know" ballot initiative is led by chemical giants Monsanto and DuPont, whose businesses also are the world's top sellers of genetically modified seeds.
Monsanto has contributed just over $7 million to fight the proposal, while DuPont has kicked in about $5 million. In all, the "No on 37" camp has raised a total of $34.6 million, according to filings with the California Secretary of State.
"Yes on 37" supporters - led by the Organic Consumers Association and Joseph Mercola, a natural health information provider - have donated just $5.5 million.
"When there's an initiative that's going to affect an industry that can rally resources, they've usually been able to stop it," said Shires. "It still could go either way."
If passed, California would join dozens of countries that already have some requirements for labeling of genetically engineered foods.
Supporters of the ballot initiative, including food and environmental activists as well as organic growers, say consumers have the right to know what's in the food they eat.
Many want GM products, which do not require premarket safety assessments in the United States, cut from the food chain.
Industry says the products are safe, but there is a fiery debate raging around the science.
Because foods made with GMOs are not labeled, it is impossible to trace any food allergies or other ill effects suffered by humans or animals, critics say.