In October, Consumers Union, the policy and advocacy arm of Consumer Reports, said that estimate was too high.

The Yes on 522 campaign raised nearly $7.9 million of its own money, including large contributions from so-called "natural" product companies, like Dr. Bronner's Magic Soaps of California.

As of Thursday morning, the website of the campaign favoring GMO labels was reporting the vote as "still too close to call." Campaign officials say they won't give up until the thousands of mail-in ballots yet to be counted are tallied. The final result will be certified by the state on Dec. 5.

A similar labeling initiative was defeated last year in California. But Maine and Connecticut passed labeling laws this summer (though both include stipulations that they won't take effect unless more states do the same).

For more on the debate over GMOs in Washington state, check out Martin's October story.

Copyright 2013 NPR.

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Washington State Says 'No' To GMO Labels