A quarrel between the House and Senate over union organizing is threatening to prevent congressional action necessary to keep the Federal Aviation Administration operating, raising the possibility of a shutdown of the agency later this week.

The FAA's operating authority expires on Friday. The agency has operated under a series of 20 short-term extensions since Sept. 30, 2007 because lawmakers have been unable to agree on a long-term funding bill.

The FAA's 15,500 air traffic controllers are essential employees and would continue to work even if the agency's operating authority were to expire, Republican lawmakers told the House Rules Committee on Tuesday.

FAA spokeswoman Laura Brown declined to comment on the agency's plans in case its operating authority expires.

However, former FAA Administrator Marion Blakey said it's likely other workers would face furloughs. The agency employs about 32,000 workers in addition to controllers. The extent of the furloughs would depend in part on how much cash is available in the federal trust fund for aviation programs and how long the shutdown goes on, she said.

Also, airlines would no longer have to collect ticket taxes, which average about $60 per round-trip ticket, aviation industry officials said.

The root of the dispute is a labor provision in a long-term FAA funding bill passed by the House in April. The Republican-sponsored provision would make it more difficult for airline and railroad workers to unionize by overturning a National Mediation Board rule approved last year that allows employees in those industries 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.

Senate Democrats, who passed their own long-term FAA funding bill in February without the labor provision, have insisted that the labor issue be removed from any final bill.

With negotiations at a stalemate, House Transportation and Infrastructure Committee Chairman John Mica introduced a bill last week to extend the FAA's operating authority for the 21st time. Previous extensions had been routine, but this time Mica added a provision to the extension bill eliminating federal subsidies for airline service to 13 rural airports.

One of the airports is in Ely, Nev., home state of Senate Democratic leader Harry Reid. Another is in Morgantown, W.Va., which is in the home state of Sen. Jay Rockefeller, chairman of the Senate Commerce, Science and Transportation Committee, which has primary jurisdiction over FAA legislation. A third is in Glendive, Mont., the home state of Senate Finance Committee Chairman Max Baucus, who has jurisdiction over the aviation tax portions of the bill.

Total cost savings would be about $8.5 million, lawmakers said.

In a letter delivered to Mica late Tuesday, Rockefeller accused the GOP lawmaker of inserting the airport provision into the extension bill in retaliation for senators' refusal to accept the labor provision.

"Your attempt to punish the Senate by hurting small community air service has backfired. This language only guarantees that the Senate will reject the FAA extension," Rockefeller said. Unless the House lawmakers agree to remove the airport language, they risk a shutdown of the FAA, he said.

Mica acknowledged that he inserted the airport language into the extension bill partly as a means to "send the Senate a message that we want this finally resolved."

The long-term funding bill approved by the Senate in February included a provision ending subsidized air service for 10 of the 13 airports that would be affected by the provision in the House extension bill. Mica said he "tweaked" that language to include three more airports -- in Nevada, Montana and New Mexico -- because their subsidies amount to more than $1,000 per ticket.

The blame for the delay belongs to Reid, who has been unwilling to negotiate on the labor issue, Mica said.

"We can resolve this in a one-hour conference," he said. "The time to stop messing around is now."