When the nuclear freeze movement was sweeping the nation less than three years ago, most of the major environmental groups elected to watch the fray from the sidelines.

Embroiled in battles over pollution and natural resources, many conservationists saw the issue as a political and moral crusade outside the scope of environmental protection.

But last month, a group of the nation's foremost environmental leaders briskly hopped aboard the antinuclear movement, calling nuclear war "the ultimate environmental threat" and urging an immediate moratorium on the production, testing and deployment of nuclear weapons.

The recommendation is at the top of an unusual manifesto by the chief officers of 10 of the nation's oldest and largest conservation groups, who dubbed themselves the "Gang of 10" and held informal meetings for more than two years to draw a blueprint for environmental activism.

The result, a book-length document called "An Environmental Agenda for the Future," urges conservationists to add a host of global issues -- from population growth and trade policy to international industrial development -- to their traditional slate of causes.

"Its purpose is to make a contribution toward getting the environmental movement to face up to global issues and to the interconnectedness of all life," said National Audubon Society President Russell W. Peterson, who spearheaded the effort. "The word we use frequently is 'holistic,' bringing a more holistic approach to bear."

But the recommendations may also reflect a growing restlessness in the organizations that have long been the bulwark of the so-called "environmental movement."

In recent years, the major national groups have frequently found themselves uncomfortably in the middle of an acrimonious debate over national environmental policy.

On one side are the prodevelopment forces represented by the Reagan administration, who are fond of labeling them overzealous "tree-huggers" oblivious to the nation's economic needs. "They won't be happy until the White House looks like a bird's nest," President Reagan once said.

On the other side is a newer brand of grass-roots activism, loosely organized around the issue of toxic pollution, whose proponents regard the national groups as "establishment" organizations whose effectiveness has been diluted by too many years at the negotiating table.

"Our Dupont Circle friends" is how Lois Gibbs, head of the Citizens Clearinghouse on Hazardous Waste, describes the national groups. "They are so used to compromise. On this issue there is no compromise. It's a political movement. It's being fought from the field and they don't fit in."

With toxic issues increasingly crowding center stage and many of the major battles for clean-air and clean-water laws behind them, the old guard appears to be searching for a broader and more dynamic role.

The restlessness is evident in the Washington offices of environmental groups, where longtime veterans have been abandoning the trenches in favor of private-industry jobs or new careers in public service.

"Everybody wants to be on the front lines," said one environmental lobbyist. "Nobody wants to be the occupation force."

Moreover, five of the major national groups have either installed new leaders in the last several months or announced they are in the market for new chief executives. Among them are the Wilderness Society, the Environmental Defense Fund and the Sierra Club, whose president, J. Michael McCloskey, has been at the helm since before Earth Day 1970.

While many in the environmental community attribute the rash of leadership changes to coincidence, they acknowledge that the national groups, as a whole, have been adopting a more conservative, less confrontational style.

Louise C. Dunlap, head of the Environmental Policy Institute, called the groups' report "a historic document" that will have a strong impact on how environmentalists' resources are allocated.

"Without abandoning their primary agenda, this will enable groups to participate on a broader range of issues," she said. "This is an agenda that goes beyond Ronald Reagan."