One of the Osprey’s biggest defenders on Capitol Hill, Representative William M. Thornberry, Republican of Texas (the aircraft is assembled in his district), said in a recent interview that the Osprey was much improved and “not where it was 5 or 10 years ago.” Mr. Thornberry also said that one of the Osprey’s biggest critics in Congress, Representative Lynn Woolsey, Democrat of California, “doesn’t have a clue what she’s talking about.”

Mr. Thornberry was referring to Ms. Woolsey’s comments on the House floor in May, when she called the Osprey “a poster child for the excesses and inefficiencies of the military-industrial complex” and offered an amendment to kill its financing. The measure failed, but an aide to Ms. Woolsey said she remained steadfast in her opposition.

Defense industry analysts say that the number of Ospreys could well be cut back from the 458 expected to be bought by the Marines, the Navy and the Air Force, but that the program is so far along it is unlikely the Pentagon or Congress will kill it entirely. (The far bigger target is the F-35 Joint Strike Fighter, the most expensive weapons program in history, although the Pentagon press secretary, George Little, reiterated Saturday that “no decisions have been made” about reductions in any weapons programs.) But at this point nearly 300 Ospreys are already in service or in production, and some $36 billion out of a projected $54 billion has been spent.

“We’ve gone this far, we may as well make the most of it,” said Mr. Aboulafia, who said the Osprey had overcome its earlier problems and was a “good aircraft,” although costly.

He credited the contractors, Bell Helicopter Textron and Boeing, but particularly the Marines, for a relentless lobbying and public relations campaign.

That campaign spanned 25 years and went into overdrive when Mr. Cheney, under orders from the first President Bush to cut spending, tried to cancel the Osprey. He said it was too expensive (at the time, the projection was $28 billion for 682 aircraft) for what he viewed as the Marines’ relatively narrow mission, amphibious assault.

But the Marines saw the aircraft as crucial to their survival as a quick-response, expeditionary force. In arguments they still make today, the Marines pressed their case that the Osprey could take off from aircraft carriers and get in and out of difficult landing zones better than airplanes and faster than helicopters, carry more people and save lives. In response to Mr. Cheney, they led a fierce counterattack, meeting with lobbyists and supporters in Congress in secret strategy sessions on Capitol Hill.