Tucked inside a huge budget bill headed for an upcoming House vote is a provision that could spur the federal government to sell off millions of acres of public land to mining interests, marking a major shift in the nation's mining policy.

The measure, which would generate an estimated $158 million in revenue over the next five years, would also put on the market key parcels of federal land in the District that had been promised to the city for initiatives such as redevelopment along the Anacostia River.

The surprise measure has angered even Republicans such as Rep. Thomas M. Davis III (Va.), who has vowed to fight it.

Congress has barred the government from selling land outright to mining companies since 1994, on the grounds that they should lease public land the same way oil and gas firms do to extract the minerals below. But House Resources Committee Chairman Richard W. Pombo (R-Calif.) said the measure would cut the deficit and promote private ownership. "In some states primarily owned by the federal government, it's important that more of that land become private property," Pombo said. "These environmental groups want the federal government to own everything."

Rep. Nick J. Rahall II (W.Va.), ranking Democrats on Pombo's committee, criticized the measure in an interview yesterday. He said that it "would result in a blazing fire sale of federal land to domestic and corporate interests."

Rahall said the government would collect hundreds of millions of dollars more if it charged an 8 percent royalty on the extracted minerals. "We're setting up Uncle Sam to be Uncle Sucker," he said.

The two sides offered sharply different estimates of how much land might be sold under the bill. The Congressional Budget Office has estimated that no more than 360,000 acres would immediately come on the market, but environmentalists said 5.7 million acres could end up on the auction block. The bill would set the sale price at as much as $1,000 an acre or "fair market value," whichever is highest -- but based only on the land's surface value, not its potential mineral wealth.

"It could be the largest privatization of federal land in the last 100 years," said John D. Leshy, a professor at the University of California Hastings College of the Law who served as the Interior Department's solicitor under President Bill Clinton.

Pombo spokesman Brian Kennedy called those estimates "patently absurd."

The legislation, which would amend the 1872 Mining Law, would allow companies to buy land they are mining and adjacent parcels for "sustainable economic development." Critics said this language, which does not require buyers to prove there are minerals beneath, could lure companies searching for real estate investments.

"Anybody who want to could go out there and make a claim on public land could just go out and do it, and do other things with the land other than mining," said Mat Millenbach, the Bureau of Land Management's former state director in Utah and Montana.

But Luke Popovich, a spokesman for the National Mining Association, said the change would provide companies with the certainty they need to pursue operations.

"You're not suddenly turning Western mining lands into high-end condominium developments," Popovich said. "These companies are mining companies, and that's far, far removed from commercial exploration."

Pombo's plan has also outraged D.C. politicians because it would undermine legislation sponsored by Davis and endorsed by President Bush that would transfer land for free to the city to compensate for Congress's ban on a commuter tax and for the resulting increase in the city's fiscal burden.

The areas affected by the bill include 100-acre Poplar Point, where the District is planning a 70-acre waterfront park surrounded by offices, shops, hundreds of apartments and possibly a professional soccer stadium; 15 acres of parking lots and fields just north of Robert F. Kennedy Memorial Stadium, where the Washington Nationals baseball team currently plays; and parcels near the Eastern Market Metro station on Capitol Hill, Waterside Mall in Southwest and the site of a new stadium for the Nationals just off South Capitol Street.

Much of the land is central to the Anacostia Waterfront Initiative, a 20-year, multibillion-dollar plan to create vibrant neighborhoods, parks, trails and cultural amenities on industrial, blighted or underused land.

"Although we understand the need to balance the budget and trim . . . we really don't think the D.C. land is the appropriate vehicle to do it," said Gregory M. McCarthy, Mayor Anthony A. Williams's deputy chief of staff for policy and legislative affairs. "We hope that on the floor, or else in conference, the D.C.-specific land stuff can come out of the bill."

Davis, whose bill is awaiting a Resources Committee vote, said he has told House leaders that he will vote against the final budget bill if it includes Pombo's language on the District. "At the end of the day, this stuff is not going to become law," Davis said of the D.C.-specific provisions.

But Pombo said the bill still gives the District half the land for free.