* Early support tumbled as opposition rose
* Passage would make it first U.S. GMO labeling requirement
* Dozens of other countries have GMO food labeling rules
LOS ANGELES/KANSAS CITY, Oct 25 (Reuters) - Heavy advertising spending by Monsanto Co and others opposed to a California ballot proposal to require labels on grocery products containing genetically modified organisms is paying off, according to a new poll that shows the measure has slipped into a virtual tie.
Forty-four percent of California voters now support the measure, while 42 percent oppose it and 13 percent are undecided, according to the latest USC Dornsife/Los Angeles Times Poll released on Thursday.
Those numbers have dropped sharply since September, when 61 percent of voters responding to the poll supported the measure, 25 percent opposed and 13 percent were undecided.
The stakes are high for both sides and with 10 days to go before the election, the state's biggest food fight this year is bound to intensify.
If California voters approve the measure on Nov. 6, it would be the first time U.S. food makers have to label products that contain genetically modified crops such as corn, soybeans and sugar beets. Scientists have spliced the DNA of those crops, and others, with DNA from different species in ways that make the crops tolerant of chemical sprays or toxic to certain insects.
The risk to companies such as Monsanto is that food companies might be more likely to stop using GMOs than to label them. That could disrupt U.S. food production because ingredients such as 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.
Monsanto and DuPont, which lead global sales of genetically modified seeds, are the top contributors to the effort to defeat California's ``Right to Know'' ballot initiative, known as Proposition 37.
The ``No on 37'' side, which also includes companies such as soda and snack seller PepsiCo Inc, has collected more than $40 million for the effort and out raised the ``Yes on 37'' camp by a significant margin, and has been blanketing the airwaves with ads criticizing the proposal.
``When voters hear a message so much more strongly from one side than the other, it's not surprising to see the poll numbers move like this,'' said Dan Schnur, director of the USC Dornsife/L.A. Times Poll and director of the Unruh Institute of Politics at USC.
The latest numbers suggest that supporters of the measure face slim odds of success.
``You never say never, but it's relatively rare for an initiative to climb above 50 percent once it has fallen below that threshold,'' Schnur said.
Stacy Malkin, media director of Yes on 37 California Right to Know campaign, said her side is up against ``nearly $1 million per day of negative deceptive advertising from the pesticide and junk food industries'' and that opponents have failed to pull ahead, despite constantly hammering California voters with ads.
``No on 37'' would not disclose its advertising spending. Spokeswoman Kathy Fairbanks repeatedly has said that the group - which among other things was forced to change an ad after it misidentified its star as a Stanford University doctor - is simply underscoring flaws in the labeling proposal.
The United States does not require independent safety testing for GM crops before they go to market.
The American Medical Association has come out in support of mandatory pre-market safety assessment for GMOs and the World Health Organization has clearly stated that labeling is perfectly valid, said Michael Hansen, senior scientist at the Consumers Union.
Industry says GMOs are safe, but a range of academic scientists, consumer and environmental and health advocates say many studies have indicated health and environmental harm tied to them. Specific health concerns include fears that the GMOs can increase the allergenicity of certain foods and possibly increase the risk of cancer or other diseases.
Dozens of countries already have GM food labeling requirements, with the European Union imposing mandatory labeling in 1997. Since then, GM products and crops have virtually disappeared from that market.
The USC Dornsife/Los Angeles Times Poll was conducted from Oct. 15-21 by Democratic polling firm Greenberg Quinlan Rosner and Republican polling firm American Viewpoint. The full sample of 1,504 registered voters has a margin of error of +/- 2.9 percentage points.