Struggling to save what has become the most controversial part of their base-closing plan, senior Pentagon officials yesterday disputed suggestions that broad changes proposed for the Air National Guard would compromise the nation's air defense.

But skeptical members of the federal commission, charged with reviewing the Pentagon plan, appeared unconvinced after a lengthy afternoon hearing intended to reconcile differences among the Defense Department and state political and military officials.

With the commission due to make its recommendations this month, several members voiced frustration after the hearing at the inability of Pentagon and Air National Guard authorities to reach a common position.

Leading Guard representatives, who also testified, reiterated concerns that the Pentagon's plan would lead to sharp drops in membership in the affected units and seriously impair national security without affording any significant savings. They offered an alternative proposal that amounted essentially to a rejection of most of the Pentagon's recommended changes.

The dispute left the commission with no apparent middle way out.

"We will solve this problem, we will act decisively," Anthony J. Principi, a former secretary of veterans affairs who heads the commission, told reporters. But he added that not having a negotiated compromise "does make our job more difficult."

The proposed restructuring would leave 29 of the Air Guard's 88 flying units without aircraft. The plan has enraged both Guard members and many lawmakers and governors, who worry their states will lack aircraft to deal with not only terrorist threats but also such natural disasters as forest fires and hurricanes.

Pentagon officials defended the proposed changes yesterday as part of a larger effort to restructure the Air Force into fewer squadrons that would be bigger and hence more efficient. In time, they said, units without planes would receive new missions such as handling remotely piloted Predator aircraft.

Peter Verga, the deputy assistant secretary of defense for homeland defense, told the commission that every state does not need military planes to ensure the protection of U.S. airspace. Adm. Timothy J. Keating, who heads Northern Command and has responsibility for U.S. air defense, testified that the proposed Guard changes would pose "no unacceptable risk" to meeting his mission.

"That's not exactly a wholehearted endorsement -- to me anyway," said Harold W. Gehman Jr., a retired Navy admiral, citing Keating's use of a double negative.

Commissioner Phillip E. Coyle, who was the Pentagon's top weapons evaluator for much of the 1990s, noted that large parts of the country would be left without fighter jets, requiring planes to fly an hour or more to establish coverage in an emergency.

Army Lt. Gen. H. Steven Blum, who heads the National Guard Bureau, sought to mollify all parties by endorsing the Pentagon plan but adding that if it were approved, he would use his authority to ensure every state would retain at least one Guard flying unit. But that personal assurance seemed to assuage neither the commissioners nor state Guard leaders.