His bill, which has 12 cosponsors in the 50-member Senate - 11 Democrats plus Sen. Stewart Greenleaf (R., Montgomery) - would require all foods sold in the state to carry the labeling information.
Genetically engineered food has been altered either chemically or through radiation, for various reasons. Among them are to encourage faster animal or plant growth, to resist pests, or to add nutrients.
According to the nonprofit Center for Food Safety, up to 85 percent of corn, 91 percent of soybeans, and 95 percent of sugar beets in the United States are made with genetically modified organisms, Leach said.
The U.S. Food and Drug Administration does not require such labeling or conduct safety studies of genetically engineered foods. Any safety consultations are voluntary, and food developers may decide what information to provide to the agency, Leach said.
"We can find out how much fat and sodium are in our food, with a full list of ingredients and nutritional information on every box, but we are not informed about the inclusion of ingredients that could be potentially detrimental to our health and wellness," he said.
Labels identifying genetically engineered food are required in 61 countries, among them Japan and South Korea, but no states require it and no action has been taken in Congress on proposals. By Leach's count, bills similar to his are pending in as many as two dozen other states.
The one that came the closest to passage was California's, where a ballot measure, Proposition 37, failed last November after opponents - among them agriculture biotech giant Monsanto, food producers, and grocery chains - mounted a $46 million campaign to defeat the measure, according to the Los Angeles Times.
Whole Foods, the supermarket chain that focuses on organic foods, announced last week that it would begin requiring that genetically engineered foods in its stores be labeled starting in 2018.
Leach, sponsor of two other controversial bills - to legalize marijuana and end the state's ban on same-sex marriage - is sure to face formidable opposition for his latest bill in a state where agriculture is the number-one industry.
The Pennsylvania Farm Bureau, a trade group for the state's farmers that has more than 50,000 members, said it would oppose the bill because such labeling could lead consumers to believe genetically modified food was not healthy.
"FDA's regulations state that requiring the labeling of foods that are indistinguishable from foods produced through traditional methods would mislead consumers by falsely implying differences where none exist," Farm Bureau spokesman Mark O'Neill said. "Farmers have grown crops from biotech seeds since 1995, and there has not been a single instance of harm to human health."
Leach argues that scientists are split on the issue, with some studies suggesting genetically modified foods have been linked to cancer and reproductive health problems in male lab mice.
"I like my testes," quipped Leach, who is known around the Capitol for using humor to deliver serious arguments. "Anything that would adversely affect them, I don't want to eat."
A number of environmental and public health groups and organizations representing small farmers and organic growers joined Leach at a news conference Tuesday to support the bill.
Leach argued that if agricultural operations and food manufacturers have nothing to hide, they should support labeling.
"Tobacco manufacturers are required to put labels on cigarettes saying the product could kill you, and people are still smoking," he said. "Dunkin' Donuts identifies doughnuts that give you six days' worth of calories, yet people still buy them."
Contact Amy Worden at 717-783-2584 or firstname.lastname@example.org or follow at inkyamy on Twitter.