Opponents have donated $17 million to defeat the referendum, which if passed would require special labels on raw and processed food made from genetically modified crops. It is the largest amount ever raised against a ballot initiative in the state.
Voters support the idea of labeling by a wide margin, according to a September poll by Seattle-based Elway Research.
The Washington state proposal is nearly identical to a 2012 California referendum that enjoyed early support but lost by 2 percentage points after a late-surging, big-spending campaign by opponents.
In that case, groups opposed to labeling, including Monsanto Co and PepsiCo, spent about $46 million on an advertising blitz.
"The money particularly comes in at the end," said Andy Behar of As You Sow, a shareholder advocacy group for environmental and social causes.
He said big food companies "should not be adding to that $17 million" in Washington state, whose population is less than one-fifth of California's. Behar spoke on Wednesday on a conference call with environmentalists who support the proposed food-labeling law.
Victory in Washington state could be a springboard for action in other states or in the U.S. Congress for the labeling movement. Food makers and biotech companies say the drive is misguided and will drive up the cost of food.
"We believe that political contributions are a poor investment and are calling companies not to spend money opposing legislation that would give consumers labeling information," said Lucia von Reusner of Green Century Capital Management, manager of environmentally focused mutual funds.
As a lever for action, Behar and von Reusner said their groups would file shareholder resolutions to prevent companies such as Monsanto from engaging in advocacy about GM labeling.
Monsanto, the largest agricultural biotech company in the world, has donated $4.8 million against the referendum.
The largest opposition donor, at $7.2 million, has been the Grocery Manufacturers Association, a food industry trade group. DuPont Pioneer, a biotech seed company, was the third-largest donor at $3.4 million.
In a statement of policy, GMA said genetically modified foods are safe and that regulators have found "no negative health effects associated with their use." It said up to 80 percent of U.S. food contains GM ingredients.
Backers of the Washington state initiative, known as I-522, had raised $5.3 million as of early October. The largest donor was Dr. Bronner's Magic Soaps, a family-run company based in California, with $1.8 million.
Connecticut in June became the first state to pass a GM labeling law. But it will not take effect unless four other states in the U.S. Northeast - with a combined population of 20 million and one of which borders Connecticut - approve similar legislation.
Maine legislators approved a labeling bill but the governor has yet to sign it.
The nationwide Just Label It campaign wants the U.S. Food and Drug Administration to set nationwide rules on GM labeling of food.
