The tactics range from old-fashioned lobbying to modern social media campaigns, and both sides say it is too early to tell who will prevail.

“I feel like we’re in the final battle now,” said David Bronner, a California business owner and leading backer of mandatory labeling for foods made with genetically engineered crops, also known as GMOs.

Bronner, CEO of Dr. Bronner’s Magic Soaps, said in a phone interview he has purchased $250,000 in advertising space in several national publications to identify what GMO critics see as concerns about GMO crops, and to challenge what he called a “smokescreen” promoted by corporations and others who say GMOs and the pesticides used on them are safe.

Bronner has already spent well over $2 million on efforts to pass GMO labeling laws in several states. His new ad buy includes advertorials in The New Yorker, The Nation, Forbes and other publications.

Leaders of the Just Label It movement say consumers want mandatory labeling so they can make informed choices about consuming foods made with GMOs.

They have been meeting with senators, armed with 50,000 signatures from consumers the group said support mandatory GMO labeling. The group said last month its “#ConcealOrReveal” hashtag reached more than 9 million people through social media.

On the other side, the Grocery Manufacturers Association (GMA), which represents more than 300 food companies, is also lobbying lawmakers hard and using social media. GMA and many top agricultural companies say GMO crops are as safe as conventional crops, and say mandatory labeling would result in unnecessary costs and confusion for consumers through an unworkable patchwork of state laws.

“This issue is a five-alarm fire for our industry,” GMA Vice President Michael Gruber said in a phone interview. “We need to get this resolved before the end of the year.”

Gruber said a U.S Senate agriculture committee is expected to take up the issue late this month. The association wants to see the Senate approve a measure like the one passed by the U.S. House of Representatives in July that creates a national, voluntary standard for labeling and blocks mandatory GMO labeling, including pre-empting a state labeling law set to take effect next year in Vermont.

Gruber said leaders from 500 different organizations supported the House measure and many are also reaching out to senators.

The debate over the safety of GMOs heated up in March when the World Health Organization’s cancer research unit classified glyphosate, the key herbicide sprayed on genetically modified crops, as “probably” cancer-causing for humans.

Glyphosate residues have been detected in food and water. GMO labeling proponents say concerns about glyphosate are a key reason GMO foods should be labeled.

Last week regulators in California, one of the largest U.S. farming states, said they are moving to list glyphosate as potentially cancer-causing. The California Environmental Protection Agency is taking public comments on the move through Oct. 5.

(Reporting by Carey Gillam in Kansas City; Editing by Jonathan Oatis)