Representatives of nine environmental groups, practicing for 1982 politics, are trying to prod two of the nation's largest health charities into helping to defend the Environmental Protection Agency.

In a letter yesterday the coalition urged the March of Dimes Birth Defects Foundation and the American Cancer Society to "telegraph your organization's concern" over EPA budget cuts to Congress, the White House and EPA through letters and private contacts.

Environmentalists argue that proposed EPA funding slashes of 18 to 36 percent for fiscal 1983 would dismantle the agency at a time of growing responsibility for toxic waste and pollution control.

Political activity by either charitable group on issues not directly linked to their immediate interests is very rare. The environmentalists, however, clearly hope to bolster the issue for elections next year.

Although tax law allows charities to spend 20 percent or $1 million of their income on lobbying, they have traditionally focused on legislation directly linked to their causes.

"A lot of what EPA does is out of our area of prevention and treatment of birth defects," said Oren Teicher, director of corporate communications for March of Dimes. He added that the charity would be "delighted to share our positions" with Congress and the White House where they are "complementary" to the environmentalists' request.

The environmental groups argue that improving the environment can prevent many diseases. "Whether it is cancer, birth defects or some other disease, we believe that each charity has a large stake in seeing an effective, competent federal agency that can protect the environment," the four-page letter said. EPA's activity, it continued, "profoundly affects the state of the public's health."

The environmentalists signing the letter represented National Audubon Society, Sierra Club, National Wildlife Federation, Natural Resources Defense Council, Center for Science in the Public Interest, Environmental Policy Center, Environmental Defense Fund, Environmental Action and Friends of the Earth.

EPA's former deputy administrator, Barbara Blum, and former planning chief William Drayton also signed, along with Gus Speth, former chairman of the Council on Environmental Quality, and James Peirce, president of the National Federation of Federal Employes.