With 92% of the vote counted, Proposition 37 was losing 53.1% to 46.9%.

Genetic engineering is a laboratory technique where scientists splice the DNA of one plant or animal and combine it with DNA from something else. The most common modifications insert genes from bacteria into crops to make them pest-resistant or able to withstand weedkillers like such as Roundup.

As biotech innovations have expanded in recent years, the percentage of crops that are genetically engineered has soared. Today, about 90% of corn and soybeans are genetically engineered, according to the USDA, as are much of the nation's canola and sugar beet crops. Those crops make their way into thousands of common food products that fill grocery stores.

Proposition 37 was the second time nationwide that voters have been asked to decide about labeling GMOs. Oregon voters rejected a similar measure 10 years ago.

Advocates concerned about potential health and environmental impacts of genetic engineering have also pushed -- unsuccessfully -- for food labeling laws in 19 state legislatures and submitted a petition to the federal Food and Drug Administration earlier this year.