Another shutdown of the Federal Aviation Administration is possible because lawmakers haven't resolved a labor issue that's holding up passage of a long-term funding bill for the agency, a key senator said Monday.

Sen. Jay Rockefeller, D-W.Va., chairman of the Senate Commerce, Science and Transportation Committee, told aviation industry officials at a luncheon that the chief holdup is a dispute between one airline and labor unions over a provision that would make it more difficult for airline workers to unionize.

Rockefeller was apparently referring to Delta Air Lines, although he didn't mention the airline by name.

That holdup is a labor provision in the House version of the long-term bill. Republicans want to overturn a National Mediation Board rule approved last year that allows airline and railroad employees to form a union by a simple majority of those voting. Under the old rule, workers who didn't vote were treated as "no" votes.

The GOP provision has the backing of the airline industry. The biggest beneficiary would be Delta, the largest carrier whose workers aren't primarily union members.

Rockefeller said there "is no movement, no give" in the GOP-controlled House. He said industry must put more pressure on Congress to pass a bill, which is critical to plans to modernize the nation's air traffic control system.

"I need your help and, frankly, I haven't been getting nearly enough," Rockefeller chastised the luncheon audience, which included dozens of industry lobbyists.

Rockefeller's House counterpart, Rep. John Mica, R-Fla., "has been and continues to be willing to reach a compromise on every single issue in the bill," said Justin Harclerode, Mica's spokesman.

Passage of a long-term bill is important to the FAA's plans to modernize the nation's air traffic control system, Rockefeller said.

If not for setbacks to the airline industry over the past decade that have held down air traffic _ an apparent reference to the Sept. 11, 2001, terrorist attacks, the high price of oil and the sour economy _ the air traffic control system would be gridlocked already, he said.

The FAA has forecast that the air traffic system will grow from nearly 800 million passengers a year today to over 1 billion passengers within the next decade.

The FAA was shut down for two weeks this summer in a dispute involving the labor issue, as well as air service subsidies for rural communities. Tens of thousands of FAA employees and airport construction workers were temporarily laid off.

The FAA's long-term operating authority expired in 2007. The agency has continued to operate under a series of 22 short-term extension bills. The current extension expires on Jan. 31.