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Major food and seed companies appear to be on the verge of defeating a California ballot initiative that, if passed on Tuesday, would create the first labeling requirement for genetically modified foods in the United States.

In a campaign reminiscent of this summer’s successful fight against a proposed tobacco tax in California, opposition funded by Monsanto Co, DuPont, PepsiCo Inc and others unleashed waves of TV and radio advertisements against Proposition 37 and managed to turn the tide of public opinion.

Four weeks ago, the labeling initiative was supported by more than two-thirds of Californians who said they intended to vote on November 6, according to a poll from the California Business Roundtable and Pepperdine University’s School of Public Policy. On Tuesday, their latest poll showed support had plummeted to 39 percent, while opposition had surged to almost 51 percent.

The swing in sentiment in the final weeks was predicted by pollsters, based on the power of a $46 million “No on 37” campaign, one of the best-funded for a California ballot measure fight. The ads claim the “badly written” initiative would increase the average family’s grocery bills by $400 annually and hobble California farmers. Opponents also take aim at what they call “special interest exemptions” for restaurant food and products from animals fed with grain containing genetically modified organisms, popularly known as GMOs.

Backers of the labeling initiative say consumers have the right to know what is in the food they eat. They dispute opponents’ cost projections and say labeling would not be burdensome to families or businesses.

They could still prevail on Tuesday if the polling turns out to be wrong, or if a last minute push by grassroots supporters takes root.

Many processed foods sold in the United States are made at least in part with corn, soybeans or other crops that have been genetically modified – crossed with DNA from other species to do things like make them resistant to insects or weed killer.