Defense officials assured a skeptical base-closing commission Thursday that the Pentagon's plan to restructure dozens of Air National Guard (search) units wouldn't pose an unacceptable risk to the country's security.

Opposing the plan, states argue that it will hinder Air Guard units from fulfilling their homeland security missions.

The shake-up of dozens of Air Guard units has emerged as the most contentious part of Defense Secretary Donald H. Rumsfeld's proposal to close, shrink or expand hundreds of military bases and other installations nationwide.

As a result, the nine-member commission reviewing the Pentagon's sweeping proposal gave the Pentagon and states one last chance to argue their cases before the panel sends a final report -- with changes if necessary -- to President Bush next month.

Rumsfeld's Air Guard plan calls for shifting people, equipment and aircraft around at least 54 sites where Air Guard units are stationed. Roughly two dozen sites would expand, while about 30 are slated for closure or downsizing. In many cases, units would continue to exist but no planes would be assigned to them. Units without planes would receive new non-flying missions.

"It poses no unacceptable risk" to homeland security, Adm. Timothy Keating, commander of the U.S. Northern Command and the North American Aerospace Defense Command (search), told the commission.

But several commissioners appeared unconvinced.

"That's not exactly a wholehearted endorsement, to me anyway," retorted Harold Gehman, a retired Navy admiral.

Peter Verga, a deputy assistant secretary of defense, said the Pentagon sought to "balance homeland defense and expeditionary needs" when it drew up the Air Guard plan. "The department focused on its ability to defend the nation as a whole ... rather than on a state-by state basis," Verga said.

The Pentagon says the Air Guard changes are part of an overall effort to reshape the Air Force -- which will have a smaller but smarter aircraft fleet in the future -- into a more effective and efficient force by putting active duty, Air Reserve and Air Guard units to work alongside one another.

But lawmakers, states and some commissioners worry about the Air Guard plan's potential impact on recruitment, retention and training. They also question whether the Air Guard will be able to fulfill its homeland security mission and whether Rumsfeld even has the authority to move Air Guard units without each governor's consent.

Two states, Pennsylvania and Illinois, have sued over the Air Guard proposal even though the Pentagon claims it has the authority to reshuffle the Air Guard as it sees fit.

The commission's legal counsel has said relocating, disbanding or moving Air Guard units from one state to another could be outside the commission's authority. The Pentagon has asked the commission to wait for a Justice Department opinion before changing Rumsfeld's plan.