This article by Marc Lifsher explains:
By more than a 2-to-1 margin, California voters favor an initiative to require food manufacturers and retailers to label fresh produce and processed foods that contain genetically engineered ingredients. With less than six weeks until election day, Proposition 37 is supported by 61% of registered voters and opposed by 25%, according to a new USC Dornsife/Los Angeles Times poll. An additional 14% were undecided or refused to answer.
With more than 10% of the U.S. population, California tends to be a regulatory trend-setter. Its regulations sometimes spur federal regulation and gently encourage manufacturers to do in other states what they must do in California. I wonder whether labeling of genetically modified foods will spread elsewhere if California adopts Prop 37 (and Prop 37 survives any legal challenges).
Prop 37 does have opponents, who claim that genetically modified food ingredients benefit consumers. Lifsher's article explains:
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.