The president stood on a sun-washed ridge in the George Washington National Forest, surrounded by trees turning shades of russet and gold, to announce details of the preservation plan, which is already under attack from Western Republican lawmakers.
"We will ensure that our grandchildren will be able to hike up to this peak," Mr. Clinton said. "We will assure that when they get to the top they'll be able to look out on valleys like this, just as beautiful then as they are now."
The forest's Little River area, comprising 27,248 acres, is among those Mr. Clinton is seeking to protect. It has high ridges and knobs that offer stunning views of the Shenandoah Valley, and is home to hardwood trees such as oaks, hickories, poplar and mountain ash.
Mr. Clinton disputed criticisms that his proposal would "turn the national forests into museums" and hurt the nation's timber supply, saying it provides future generations of Americans with a treasure more valuable than timber.
"National forests are more than a source of timber. They are places of renewal of the human spirit and our natural environment," Mr. Clinton said. "We want this for our children forever."
Earlier this year, Mr. Clinton proposed a $1 billion "land legacy" initiative to purchase open spaces, but that has been largely thwarted by Republicans in Congress, who refused to fund it.
Mr. Clinton urged Congress to approve that plan, and said he would reject any proposal that comes to his desk with anti-environmental riders on it. "I will send it straight back to the recycling bin," he said.
The forest protection plan would require no congressional action, relying on regulations to be issued by the U.S. Forest Service after a detailed environmental review and public comments.
In addition to the 40 million acres, Mr. Clinton asked the Forest Service to determine whether 15 million pristine acres still being inventoried should be protected, said White House spokesman Joe Lockhart.
Currently, about 18 percent of the 192 million acres of federal forest is protected as wilderness. About 60 million acres are without roads, or signs of logging, mining and other development.
Mr. Clinton's plan would cover isolated forest areas of 5,000 acres or more and would affect road-building and other development in 35 states, most of them in the West.
"It would be one of the most significant land conservation actions by the United States government in its history," said Richard Hoppe, a spokesman for the Heritage Forests Campaign, a coalition of conservation groups that has pressed for protection of roadlss forest areas.
"The president's trying to be Teddy Roosevelt," snapped Rep. Bob Goodlatte, R-Va., in whose district Mr. Clinton made the announcement. He asserted that Mr. Clinton is trying to please environmentally conscious voters at the expense of preserving healthy forests.