SACRAMENTO, Calif. — The cereal aisle, the cookie shelf and the cold case full of sodas have become the battlegrounds in one of the most expensive political fights on California’s November ballot.

Under Proposition 37, food containing genetically engineered ingredients would have to be labeled as such, including thousands of packaged products that now fill grocery stores and routinely make their way into Californians’ kitchens.

Those on both sides of the measure agree: Almost anything that’s not organic and comes in a box, a bag or a can probably has some genetically engineered content. The prevalence of such ingredients, also known as genetically modified organisms – or GMOs – is the reason advocates are calling for labels.

Common ingredients in processed food – including corn syrup, sugar, canola oil and soy-based emulsifiers – now often come from crops that have been genetically engineered. Those ingredients are in cereals, sodas, cookies, crackers, salad dressings and many other packaged foods.

Certified organic goods, on the other hand, do not contain crops whose genes have been altered in a lab.

So it’s not surprising that Proposition 37 is shaping up as a battle between organic farmers and food manufacturers on one side and, on the other, conventional grocery store brands and the biotech companies that make some of their ingredients.

The parent companies for Cheerios, Chef Boyardee, Nestle, Coke and Pepsi are pouring millions into defeating the measure. With them are companies such as Monsanto, DuPont and Bayer that make pesticides and genetically modified seeds. Thanks largely to those businesses, the “no” side has raised $32.5 million.

Pushing for Proposition 37 are organic farmers, advocates and food manufacturers such as Lundberg’s, Nature’s Path, Clif Bar and Amy’s Kitchen. They have helped the “yes” side raise $4.3 million.

Proponents contend that genetically engineered food is unhealthy and say the labeling requirement is necessary so people know what they’re eating. Opponents say genetically engineered food is safe, and that the labeling scheme described in Proposition 37 would confuse shoppers and raise the price of groceries by nearly $400 a year for the average household.

The projected increase assumes many companies would swap out low-cost genetically engineered ingredients for higher-priced unmodified ones in order to avoid the labels.

“I may not be typical, but I’m willing to pay more for food that I think is healthier for me,” said Tara Crowley, a retired university administrator who was shopping recently at the Sacramento Natural Foods Co-op, where she picked up a $3.19 package of organic whole wheat pasta to go with the fresh basil in her basket.

Donna Ridley had other priorities when she walked into a Sacramento Safeway the next day. Ridley, who described herself as a veteran with post-traumatic stress syndrome, said she lives on public assistance. She was focused on one thing as she scanned for something for breakfast: price.

Ridley picked a box of four frozen bacon, egg and cheese sandwiches because it was on sale for $5.99. Corn syrup, which usually comes from genetically modified corn, was listed as an ingredient.

Ridley said she didn’t like the idea of eating biotech food.

Would she buy the same product if it carried the label required under Proposition 37, “Partially produced with genetic engineering”?

“If it was on sale, I might buy it anyway,” she said. “Because I just don’t have that much.”

At the heart of the fight over Proposition 37 is a technology that has exploded in the last two decades, in which scientists change the DNA of a crop to give it different characteristics. Common genetically engineered crops include corn infused with pesticide to make it resistant to bugs, and soybeans bred to tolerate herbicides such as Roundup. Crops can be engineered to withstand drought, delay rotting or add vitamins.

Such biotechnology is now so commonplace in the United States that about 90 percent of the nation’s corn and soybeans are genetically engineered, according to the U.S. Department of Agriculture – a huge increase from just a decade ago. It’s a concern to proponents of organic farming but good news for producers who can grow greater quantities at lower cost.

The World Health Organization says the genetically engineered food currently on the market is safe, and the American Medical Association says there is no reason to specially label it. Hundreds of health studies vouch for the technology.

That leads Colin A. Carter, a University of California, Davis, agricultural economist who conducted an independent analysis of the proposition, to view the debate as more about the business of food than its safety.

He predicts that more people would buy organic goods if comparable non-organic items carried labels saying they’ve been genetically engineered.

“This does not present a health risk,” Carter said. “It’s about money.”

Proposition 37 backers say they just want to give consumers choice. If genetically engineered food is safe, they argue, then why is there such opposition to labeling it?

They are skeptical of the technology, citing studies that link genetically engineered foods with allergies, and a National Academy of Sciences report that says such foods could introduce “unintended compositional changes that may have adverse effects on human health.”

The long-term effect of eating genetically engineered food is unknown, said Grant Lundberg, an organic rice farmer in Butte County who supports Proposition 37. He said it’s not about creating new customers.

“I don’t see that we’re going to end up with a big chunk of new business from this,” Lundberg said. “This is about transparency, the consumers wanting to know. And the market will bring to market the things consumers want.”

Rice cakes and other snacks produced by Lundberg’s company carry a voluntary “Non-GMO” label indicating the items don’t contain genetically modified ingredients.

The same label was on the $4.79 box of organic cereal Mary Cargile plucked off the shelf at the Natural Foods Co-op.

“I’m not inclined to eat anything engineered,” she said. “It’s not something I look for, but I’m always really happy to see it when they say it’s not happening.”

The co-op and Whole Foods have endorsed Proposition 37, but most grocery stores are opposed. Ronald Fong, president of the California Grocers Association, said the measure makes it too easy for people to sue stores selling unlabeled food.

James Wheaton, one of the key lawyers behind Proposition 37, also backed Proposition 65 in 1986, which allows people to sue businesses that expose people to cancer-causing chemicals if they don’t post warnings about them.

“We see Prop. 37 going down that same path, but even a little bit worse,” Fong said.

“Trial lawyers that plan to sue on Proposition 37 might not know where their box of Cheerios is made, but they sure as heck will know where they bought it. So retail grocery stores are going to be on the front line of the lawyers when they come around.”

How much do people look at food labels anyway?

Generally, not very much, said Christine Bruhn, a consumer behavior expert at UC Davis. But it’s likely the term “genetically engineered” would scare away a lot of customers, she said, citing a study that asked how people would respond to a loaf of bread if its label said it contained wheat genes.

“Half the people would believe their bread was less safe or they were uncertain about it,” she said – even though all wheat bread contains wheat genes.

Adding a “genetically engineered” label to common foods would mislead people to think they were unsafe, Bruhn said.

Such labels are required in more than 40 countries, Proposition 37 supporters point out, including much of Europe. They say food companies should give Americans the same choices.

But food researchers say very few items in Europe actually bear the label. Rather than adding information that could frighten customers, most companies selling food there use ingredients that aren’t genetically engineered.