With increasing alarm, Northern Virginia officials are eyeing a national debate on shutting U.S. bases and shuffling other military facilities, saying the region could be hit hard by what one defense analyst has dubbed the "mother of all base-closing rounds."

Fort Belvoir in southern Fairfax County is among those major facilities that almost certainly would be spared in any downsizing, but local officials say they are less sure about the large and more amorphous military presence in private office buildings throughout the region.

"Most people have focused on base closings and haven't looked at the other side of the coin," said Fairfax County Board of Supervisors Chairman Gerald E. Connolly (D). "We need to collectively target our resources to respond to this potential threat."

Secretary of Defense Donald H. Rumsfeld has pushed an ambitious and controversial effort to shrink what Pentagon officials say is a marked surplus in military space. The department has "24 percent excess installation capacity," according to a March report issued by the Pentagon as part of the process of choosing facilities to close or relocate.

But closing bases is just part of the effort to more efficiently accommodate defense workers and save billions of dollars, said Loren B. Thompson, a defense specialist at the Lexington Institute, a conservative think tank in Arlington.

"You can't really get the full savings unless you think through where you're doing the work and how you're organized to do it," Thompson said.

Thompson added that he is not surprised that local officials are beginning to agitate on the issue. Thousands of military workers in Northern Virginia locales such as Crystal City and Baileys Crossroads could end up being moved for financial or security reasons, he said.

"There's money and jobs on the table, and people are looking to protect their parochial interests," Thompson said. "The reality is, almost no other place in America has as many federal jobs as Northern Virginia, so it can probably spare them better than other places."

Officials from throughout Northern Virginia were briefed late last month by Barry Steinberg, the Army's former chief litigator, who now works for a Washington law firm that specializes in fighting military closings. The briefing was held for the Northern Virginia Regional Commission, made up of elected leaders and other residents of more than a dozen cities, counties and towns. Steinburg painted a stark picture of potential losses in Northern Virginia, participants said.

"It caught a number of us somewhat off guard," said G. Mark Gibb, the Regional Commission's executive director.

According to the commission's analysis of a General Services Administration list presented at the meeting, the military has about 6.8 million square feet of office space in Northern Virginia commercial buildings. Leases are concentrated in Ballston, Rosslyn, Springfield and the Skyline development in Fairfax, among other places. Officials believe the analysis undercounts the total and are seeking additional data, Gibb said.

"In other areas of the state, local governments are much more organized to protect their facilities," said state Sen. Linda T. "Toddy" Puller (D-Fairfax), an appointee to the Virginia Commission on Military Bases. Gov. Mark R. Warner (D) formed the commission last year to protect the commonwealth's military facilities.

In areas such as Norfolk and Newport News, where defense installations dominate communities, officials have been busy analyzing the economic effect of bases and lobbying Virginia's congressional delegation.

In selection criteria put forward by the Department of Defense in February, military value is supposed to trump community impact. The process of choosing which facilities to shutter or shift is designed to minimize the sway of politicians. But previous efforts have been influenced by political considerations.

Some in Congress have sought to delay or derail the latest round of base closings; others contend that those seeking to stall closings often fuel unreasonable fears with exaggerated warnings. Decisions on particular sites are scheduled to be made next year, though the House of Representatives has moved for a two-year delay.

"I don't know that anything's potentially going. We're actually growing in the region, with regard to federal employment," said Rep. Frank R. Wolf (R-Va.). If closings appeared on the horizon, "the region would come together to protect the region," he said.