The Clinton administration, determined to establish a significant conservation legacy, will announce next week an initiative to protect as much as 40 million acres of national forest land from commercial development.

The move will take the form of a directive from President Clinton to the U.S. Forest Service to prepare an environmental analysis of how to best conserve the agency's inventory of "roadless" or undeveloped areas in scores of national forests across 35 states, according to sources inside and outside the administration. Much of the acreage is in the West, concentrated in the Rocky Mountain states and California.

The scale of the proposal would make it one of the most significant land preservation undertakings in U.S. history, extending protection to an area equal in size to Virginia and West Virginia combined.

By comparison, the nation's total inventory of congressionally designated wilderness parcels set aside over the past 35 years since passage of the Wilderness Act is a little more than 100 million acres. The entire national forest system comprises 192 million acres.

"If done right, this would be a legacy to rival Teddy Roosevelt's," said Nathaniel Lawrence, a senior attorney with the Natural Resources Defense Council.

"This could be truly historic," agreed Ken Rait, director of an environmental consortium known as the Heritage Forests Campaign. "America's open spaces and wild places are shrinking day by day and this would be an incredible and historic move to save these places for future generations."

Forest protection advocates believe preservation can be accomplished administratively without congressional approval, just as the Clinton administration devised an overall resource protection plan for national forests in the Pacific Northwest to protect endangered species.

"They are doing through a regulatory process what they can't do legislatively," said Michael Klein, a spokesman for the American Forest and Paper Association. "They don't have the votes, so they are doing an end-run around Congress to jam this elitist policy down the throats of the American people." The administration plan would involve preparation of an environmental impact statement with different land management options, selection of a "preferred alternative" and then issuance of a final record of decision. The result would be subject to legal challenge, and could be overturned by Congress.

"If the Clinton administration is seeking a legacy with this announcement, it will be a legacy on the cheap because they haven't done the heavy lifting to find a balance between competing views on resource management," said Sen. Frank H. Murkowski (R-Alaska).

It is not clear what specific activities would be permitted on the lands in question. But it is likely the administration will aim to give the Forest Service's roadless areas significant protection as wild lands. That designation would not prohibit as many kinds of activities as would designating the forests wilderness areas in which logging, mining, construction of structures and all motorized equipment are banned.

"The obvious things to focus on are the most harmful activities: road-building, logging, mining and off-road vehicles," said Lawrence. "How much of that they bite off is an open question."

A White House official said yesterday that the timing for announcing the proposal and some of the substantive details are still under discussion. "There are several options under consideration," said the official. "There have been no final decisions." The Forest Service, under chief Michael P. Dombeck, has been moving toward broad prohibitions on road construction and logging in areas that are still roadless. In March, the Forest Service imposed an 18-month moratorium on road construction across 33 million acres of forest while the agency develops new policies on managing its vast network of roads, which totals 380,000 miles -- eight times the length of the interstate highway system. And last February, in a speech in Missoula, Mont., Dombeck said, "It is my expectation that in the future we will rarely build new roads into roadless areas, and if we do, it will be in order to accomplish broader ecological objectives."

The significance of the pending White House announcement -- which could be made by Clinton next Wednesday during a visit to the George Washington National Forest in Virginia, a senior White House official said -- is that it puts the president's imprimatur on the development of a conservation policy for roadless areas and sets that policy review in motion.

Keeping wild forest parcels free of roads is considered by many conservationists to be the key to protecting them. Transportation corridors disrupt wildlife, facilitate logging and other commercial activities, and degrade pristine areas through erosion and other effects. Roadless areas also tend to be important refuges for imperiled animals such as grizzly bears.

But the nation's remaining roadless areas also contain some of the Forest Service's most commercially valuable timber stands. Any attempt by the White House to take administrative action to close millions of Forest Service acres to the timber industry will be strongly opposed by the industry and its congressional allies.

Western Republicans from states, such as Idaho and Montana, that have not yet resolved their Forest Service wilderness area selections might also view the action as the creation of de facto wilderness and a usurpation of congressional authority.

The political sensitivity of the issue is evident as senior administration officials continue to debate how broad a net they should cast. A key unresolved question, for example, is whether to include the Tongass National Forest in Alaska in the environmental review.

At 17 million acres, the Tongass is the nation's largest national forest, and in many ways its most controversial. The environmental community believes that excluding it from the roadless areas review would undermine the credibility of the entire proposal. But including it would likely infuriate Sen. Ted Stevens (R-Alaska), the chairman of the Senate Appropriations Committee.

At the same time, excluding it could hand former New Jersey senator Bill Bradley an environmental issue to use against Vice President Gore as they compete for the Democratic presidential nomination, because Bradley has a record of advocacy on behalf of the Tongass and has recently been endorsed by one national environmental group.

Asked whether the Tongass will be included in the White House initiative, a senior White House official said yesterday: "There's a whole range of options there that we haven't worked out."

"These guys are getting cold feet on the Tongass," said Matt Zencey, who heads up a pro-Tongass advocacy group, the Alaska Rainforest Campaign. "It's another example of where people have had high expectations for Gore and he doesn't meet them. It allows Bradley to run to his left on the environment."

The Tongass was left out of the 18-month road moratorium, as were about two dozen other forests that also have recently updated their long-term management plans or are covered by the Clinton administration's Pacific Northwest forest plan. It is expected that the Pacific Northwest forests would be included in a proposed scientific review.

Staff writer Charles Babington contributed to this report.