By David Alexander

WASHINGTON (Reuters) - The U.S. Air Force has a quarter of the number of fighter squadrons it did 25 years ago and two-thirds of the active duty airmen, a drop that threatens U.S. air superiority, defense officials told lawmakers on Friday.

"Enough is enough," Air Force Secretary Deborah James told lawmakers in the House of Representatives as she defended a Pentagon budget request that exceeds federal spending caps. "Given the state of the world ... the number one thing we have to stop is this downsizing."

But members of the defense appropriations subcommittee said President Barack Obama's 2016 Pentagon base budget of $534 billion exceeded spending caps by nearly $35 billion and would have to be cut. Some $10 billion of that would have to come from the Air Force request, they said.

"The budget he (Obama) submitted ... frankly is politically ... a fantasy," said Representative Tom Cole of Oklahoma. "It's not going to pass, and he knows that."

Cole said he hoped lawmakers ultimately would be able to reach a bipartisan deal to provide some relief from the spending limits.

Until that happens, however, "we're going to have to live within the numbers that we have under the law, and sadly, that means we're going to have to make a lot of tough choices" about cutting billions from the Air Force spending request, he said.

Air Force officials said the force is older and smaller than it has ever been, and after 14 years of continuous warfare is being stretched to the breaking point.

"When we deployed to Operation Desert Storm in 1990, our Air Force had 188 fighter squadrons in the inventory. This budget will take us to 49," said General Mark Welsh, the Air Force chief of staff. "There were 511,000 active duty airmen during Desert Storm. We have 200,000 fewer today."

James said Air Force planes had an average age of 27 years, and Welsh said updating the fleet was imperative.

"The option of not modernizing isn't really an option at all," Welsh said. "Air forces that fall behind the technology curve, and joint forces without the full breadth of air, space and cyber power ... will lose."

James said if the Air Force had to cut another $10 billion, it would reduce purchases of Lockheed Martin's F-35 Joint Strike Fighter by 14, cancel work on a fuel-efficient engine and reduce the number of reconnaissance aircraft in service.

(Reporting by David Alexander; Editing by Alan Crosby)