Leach's bill is patterned after California legislation that was narrowly defeated in 2012 after much wrangling and huge sums of money - $46 million spent by industry and other labeling opponents, $9 million by labeling supporters.
If the California experience is any indication, the fight in Pennsylvania promises to be heated and expensive.
But anyone looking for definitive, independent evidence about health effects or other concerns isn't going to find much.
Labeling initiatives are gaining traction. Similar bills have been introduced in several other states. Whole Foods announced two weeks ago it would mandate labels on all foods containing genetically engineered ingredients in its supermarkets nationwide by 2018. Sixty one countries already require it.
The industry supports voluntary labels as a marketing tool to help consumers find these foods if they want them. But requiring a label could falsely suggest to shoppers that genetically engineered foods are less safe or nutritious, said Karen Batra, a spokeswoman for the Biotechnology Industry Organization.
Pro-labeling groups say numerous studies shed doubt on the safety of foods that have been genetically engineered to add or enhance certain properties.
The industry says much of that research is flawed.
One in particular, a 2012 study from France that concluded rats fed genetically engineered corn were more likely to develop tumors, was widely debunked after publication in Food and Chemical Toxicology, a peer-reviewed journal.
Studies done or funded by the industry are inherently suspect.
So executives point to American Medical Association and World Health Organization statements that there's no evidence of health risks.
But a 2004 report issued by the National Academy of Sciences and cited by the industry concluded federal agencies should assess the safety of genetically engineered foods on a case-by-case basis because scientists had limited ability to predict health effects.
"The bottom line here is . . . there are unanswered questions begging for unbiased research," said Sam Bernhardt, Pennsylvania organizer for Food and Water Watch, a nonprofit that supports labeling.
That's the basis of Cantrell's worries. "There have been no long-term, independent, peer-reviewed studies on the health risks for humans," she said. Results of animal studies leave her with "huge concerns."
Given that about 90 percent of all corn, soybeans, and sugar beets grown in the United States is genetically engineered - and that about 70 percent of processed food in grocery stores contains at least one genetically engineered ingredient - it's not a minor question.
Besides confidence in its safety, the industry says the technology holds much promise for human health and the environment.
For instance, companies are in the end stages of developing a soybean with significantly less saturated fat and zero trans fats.
That, in turn, may help domestic farmers recover sales of soybean oil they lost to less-fatty palm oil and canola oil from overseas, said Sam Wellman, a Nebraska farmer who chairs the American Soybean Association.
Corn that has deeper roots to help it withstand drought is on the way.
"This is the cutting edge . . . crops able to withstand some of the environmental stresses we're starting to see with climate change," Batra said.
At the same time, wide use of herbicide-resistant crops has led to "superweeds" that won't die without stronger chemicals.
Some farmers worry that genetically engineered seeds, which, unlike regular seeds, are patented and controlled by major corporations, will make farming more expensive. (A counterargument: The crops they produce have other benefits that will make farming more profitable.)
One of the people who watched from the sidelines during the fracas over the California measure was molecular geneticist Michael Eisen.
The hyperbole being slung by both sides dismayed him, and he described the status of the research as a "total mess."
"Basically, everybody doing the research has a vested interested in the answer," said Eisen, who teaches at the University of California, Berkeley. "I certainly don't trust Monsanto to study the effects of their products," he added, but a large part of research also is being done "by people who decided long ago" that genetically engineered foods were unsafe.
What Californians were left with was "two sides with lots of money shouting things that were not relevant," he said. "I hope Pennsylvania doesn't go through the same problem."
GreenSpace: If You Want to Avoid Genetically Engineered Food
In the absence of labels, and with genetically engineered ingredients ubiquitous in food sold in the United States, it is nearly impossible to be sure of what you are getting.
Terminology further complicates matters. GMO, for genetically modified organism, is often used interchangeably with genetically engineered. But "modified" could apply to all crops, which have been cross-bred for certain traits for centuries.
Experts say taking these steps will make it more likely your food has not been genetically altered:
Buy unprocessed foods.
Buy fresh. Most fruits and vegetables have not yet had genetic engineering.
Beyond that, says Annmarie Cantrell, a member of GMO FREE PA, consumers should get to know their food supply better by shopping at farmers' markets, joining community-supported agriculture farms, and making "face-to-face connections with the farmers."
Contact Sandy Bauers at 215-854-5147, email@example.com, or follow on Twitter @sbauers. Read her blog, "GreenSpace," at www.philly.com/greenspace.