Seattle - Monsanto Co. and DuPont Co., among the biggest makers of bioengineered crop seeds, are persuading Washington state voters to change their minds about a proposal to require labels on genetically modified food.

The companies are backing an anti-labeling campaign with $18.1 million - twice that of advocates for a ballot measure next month. The labeling proposal had a 45 percentage-point lead among registered voters five weeks ago that has narrowed to 4 points since opponents began advertising, the independent Elway Poll showed Oct. 21.

"This is a David and Goliath fight," said Trudy Bialic, a spokeswoman for PCC Natural Markets in Seattle. Store shelves are lined with tags - "Non-GMO Project verified product"- on products from canola oil to granola bars, to reassure those who fear genetically modified organisms. "There's no way we can compete with the resources of Monsanto, Dow and DuPont."

Washington joined 26 states with proposals this year to mandate such labeling or to prohibit genetically engineered food, according to the Center for Food Safety, a nonprofit environmental advocacy group. If voters approve Initiative 522, Washington would be the first to require labels. While Connecticut and Maine have passed labeling laws, they won't take effect until more states do likewise.

PCC, whose nine stores make it the largest consumer-owned natural food retail co-operative in the U.S., donated $423,174 to support the proposal. "Vote Yes on 522!" is at the top of its website and signs urging support are posted in store windows.

Monsanto, the world's biggest seed producer, contributed $5.1 million to oppose the measure as of Oct. 2, according to MapLight, a nonpartisan research organization based in Berkeley, Calif. That compares with $1.53 billion that the St. Louis- based company spent on research and development in the year that ended in August, when sales reached $14.9 billion.