The Humane Society and other animal rights groups say King's amendment goes too far, especially because it would apply to all agricultural products and not just eggs.
"We think in effect it will nullify state laws that address mistreatment of farm animals," said Howard Goldman, the group's Minnesota state director.
Goldman doesn't expect Americans to give up eggs or bacon. "We're not saying there should be no animal slaughter. We're saying animals should be raised humanely and slaughtered humanely."
The animal livestock industry has rallied behind King's amendment and is actively lobbying the 41 members of the House and Senate conference committee to get the measure in the final bill.
“The King Amendment allows our farmers to produce their eggs and to market them into other states based on the way we produce, the production practices we have,” said Steve Olson, the executive director of the Chicken and Egg Association of Minnesota.
Producers worry their production methods could also be regulated more tightly by legislatures in other states, said Roger McEowen, law professor at Iowa State University.
"It could be free range eggs today, free range pork tomorrow, free range beef the next day,” he said, “whatever the case is."
McEowen argues that California's law is on shaky constitutional ground. The Commerce Clause of the Constitution prohibits states from setting up barriers to interstate commerce.
"For states to do this on their own, unless they've got some kind of legitimate reason to do so to protect the health and welfare of the residents, they can't do it," McEowen said.
Drake University law professor Neal Hamilton isn't so sure. He said the question is whether laws such as California's are designed to favor locals over those from out of state.
"There was no discriminatory intent to try to favor local farmers at the expense of people in other states,” Hamilton said.