Both measures would require labels on foods made with genetically engineered crops, which are common in the United States. Voters in Colorado and Oregon weigh in on the issue in mid-term election voting on Nov. 6.
Similar mandatory labeling measures failed at the ballot box in California in 2012 and in Washington state in 2013. Vermont passed a mandatory labeling law this year, but a group of grocery manufacturers has sued to block the law.
Opponents who have contributed to defeat the measure include a long list of corporate powerhouses such as Monsanto Co., the leading developer of biotech crops. Other large contributors include PepsiCo Inc.,, Kellogg Co. and Kraft Foods.
Campaign finance reports filed on Wednesday in Oregon show opponents have pooled about $10.7 million to try to defeat the measure. That compares to more than $5.5 million from supporters of mandatory labeling.
In Colorado, many of the same labeling opponents have contributed more than $11.2 million, versus roughly $441,000 contributed by supporters of labeling, according to campaign reports filed this week.
Among the supporters of GMO labeling is the ice cream company Ben & Jerry's, which is also one of the major financial backers of the effort. Other supporters include food safety and family farm groups, organic food companies and individuals.
Larry Cooper, co-chair of the coalition pushing for passage of GMO labeling in Colorado, said he believes foes of labeling want to fool consumers.
"For them to put in more than $11 million, obviously they think it is important to try to fool Colorado voters," he said.
Colin Cochran, a spokesman for the opposition campaign in Oregon, said a requirement to label GMO foods would confuse consumers and add costs to food products.
"It won't really help voters with their food choices," said Cochran. "It misleads consumers."
Supporters of GMO labeling say foods made with GMO ingredients can be harmful to human health due to pesticide residues and the altered crop genetics, and consumers deserve to know if the food they eat are made with gene-altered corn, soybeans, sugar beets and other biotech crops.
But opponents of labeling say genetically modified crops are as safe as conventional crops, and labeling would be a costly and logistically difficult burden on food manufacturers.