According to the letter, two television stations in Charleston, S.C., share news staff who read from the same scripts. Viewers in Honolulu see the same news content on three stations.
"We urge you to take account of how the reduction in local broadcast competition harms local communities and markets, and to ensure that neither the substance nor the goals of the media ownership rules are thwarted," the groups wrote.
The FCC is currently reviewing its media ownership rules.
The organizations that signed the letter are Time Warner Cable, Dish Network, the American Cable Association, Free Press and two unions that are members of the Communications Workers of America.
The National Association of Broadcasters defended the practice of operation-sharing.
"Evidence shows that when a strong local TV station shares resources with another broadcaster, the result is the creation of more local news, weather and sports," NAB spokesman Dennis Wharton said in a statement. "The simple reality is that newsgathering and public affairs programming costs money, and that viewers benefit by more choice from a TV station that is free to the public."
Wharton took particular aim at media reform group Free Press for signing the letter.
"If the goal of Free Press is to eliminate competition that local broadcasters provide to pay TV conglomerates like Time Warner Cable and DISH, perhaps it should change its name to Pay Press," he said.