Voters in California will decide on a proposal next month that would require the labelling of most foods made with genetically modified ingredients.
Proposition 37 is supported by the organic industry but many major food suppliers oppose it saying it will drive up prices.
Around $40m is expected to be spent on campaigning with the majority coming from opponents.
But a recent opinion poll shows a clear majority in favour of the proposal.
There have been attempts in 18 states to change labelling laws on genetically modified foods via legislation. None have been successful.
But after a grassroots campaign that garnered more than a million signatures, the measure will be on the ballot paper in California for the first time next month.
If passed, the statute will require labelling on raw or processed food offered for sale to consumers if the food is made from plants or animals with genetic material changed in ...