Prop 37 would require labels on all foods containing genetically-modified ingredients.
The California Business Roundtable and Pepperdine University poll shows support has declined by 19 points in the last two weeks. It's dropped from 67-percent to 48 percent.
Michael Squires with Pepperdine says one reason for the change might be that 43-percent of voters say they've seen advertising about the measure.
SQUIRES: "It's always hard to attribute specifically whether it's learning, it's them doing their homework, or whether it's the media campaigns, but clearly you have to believe given the amount of advertising that's been going into it that's got to be one of the factors."
The "No on 37" campaign has spent about 19 million dollars to oppose labeling GMO ingredients.
Voter opposition to the measure increased from 27-percent to 40-percent in two weeks.