Opponents of the food labeling argue it will raise food prices and hurt farmers. Supporters contend that consumers should have a choice about eating genetically engineered products, even if the federal government and major science groups say such foods are safe to eat.

Proponents promised to take their fight to the Northwest after the California ballot measure failed.

On Thursday, initiative sponsors delivered 350,000 petition signatures to state officials inside an ambulance with a sign on the side reading "Label GMO Food."

To qualify for the ballot, it requires at least 241,153 signatures of registered state voters, though the secretary of state's office suggests collecting at least 320,000 as a buffer for duplicate or invalid signatures.

Initiative 522 would require food and seeds produced entirely or partly through genetic engineering and sold in Washington to be labeled as such, beginning July 1, 2015. Raw foods that are not packaged separately would have to be labeled on retail shelves.

Supporters say consumers benefit from having more information.

"Yes, you can steer clear of certain items, but unless you know that they're there, how do you know to steer clear of them?" asked Chris McManus, the initiative sponsor and owner of a small advertising firm. "Putting a label on the front of that just informs the consumer a little bit more about what they're buying."

The nation's food labeling system already is built around giving consumers information about health and safety, countered Heather Hansen, executive director of Washington Friends of Farms and Forests.

"We think this is really intended to be a scare tactic, to ultimately scare people away from technology," she said. "And it's not providing any meaningful information."

Once the proposal goes to the state Legislature, lawmakers have the option to vote on it, take no action and send it to the November ballot, or recommend an alternative measure that will appear on the ballot with it.

About 50 countries require genetically modified foods to be labeled, but the U.S. isn't one of them. Only Alaska has enacted legislation requiring the labeling of genetically engineered fish and shellfish products.

A bill in the Washington Legislature to require food labeling failed to pass last year, despite support from a coalition of wheat farmers who said they feared their export markets would be hurt if genetically modified wheat gains federal approval.

Biotech giant Monsanto Co. has announced plans to begin testing genetically modified wheat, though the product is likely a decade or more from being offered commercially.