But the large industry-wide unions like the Association of Flight Attendants and the Air Line Pilots Association, which represents pilots at American Eagle and at US Airways' regional carriers, PSA and Piedmont, may compete for members.
"The big question for the pilots and the flight attendants is going to be how, in this big group, does it make sense for me to remain independent or to have a national union represent my interest," said Bill Swelbar, an airline researcher at the Massachusetts Institute of Technology.
A fight for the hearts
Not all the unions at American and US Airways have been cheering for a merger.
The International Association of Machinists and Aerospace Workers withheld its support for the merger, saying it wants new contracts instead for the 8,600 ground crew, mechanics and related workers it represents at US Airways.
"We have been in contract talks for about two years, and we want contracts for our members before we discuss any merger-related issue with US Airways," Machinists union spokesman James Carlson said. He said members want wage increases and better health and retirement benefits.
The Machinists have not signed a memorandum of understanding. The union's counterpart representing American's mechanics and ground workers, the Transport Workers Union, has signed a a memorandum, but not all those mechanics are pleased with it or with the contract their union negotiated.
American's mechanics approved a concessionary contract in August by a margin of only 48 votes out of more than 9,500. In its memorandum of understanding with American, the union agreed to drop a profit-sharing agreement in the new contract for a 4.3 percent raise.
"We're missing a bunch of [the Machinists' current] contractual items," transport workers Local 565 President Gary Peterson said of his union's memorandum. "My guys would rather have [the union's International President] Jim Little go in and say we want what US Airways already has."
For example, US Airways mechanics have 10 holidays that earn double time, whereas American's mechanics have five holidays paying time and a half.
"That is just one of many contractual items that TWU members want back," said Peterson, who represents line mechanics at Dallas/Fort Worth Airport.
Complicating the mix are two other unions, the International Brotherhood of Teamsters and the Aircraft Mechanics Fraternal Association. Both are courting American mechanics in an effort to unseat the Transport Workers Union.
"American Airlines mechanics came to us and said, 'We want new representation,' and they have been working very hard since June of last year to make that a reality," Teamsters spokesman Chris Moore said. The Teamsters said this month that they have signed representation cards from a majority of mechanics at several maintenance stations in American's network, including Dallas/Fort Worth.
The National Mediation Board requires 50 percent of eligible employees in a work group to back a representation change before it will hold an election. Moore said the Teamsters are very close to that mark and expect an election before the board awards single-carrier status to American-US Airways.
The Teamsters are mounting a similar campaign at US Airways to unseat the Machinists, but Moore declined to comment on its progress.
The Transport Workers Union has started fighting back, sending mailers and brochures to its members contrasting its union with the Teamsters.
"If you really want to study Teamsters' representation in the aviation sector, buy a plane ticket to China, where most of United Airlines' aircraft are repaired," Little said. "In contrast, American Airlines repairs the bulk of its aircraft in-house and in the United States, and will continue to so under agreements reached by TWU with both AMR and US Airways."
While it is rare for workers to leave one union for another, airline analysts say it's easy to find disgruntled workers after a concessionary contract signed during bankruptcy.
"You're forced to participate and then you have sharks swimming around, pointing at the fact that you gave concessions," Swelbar said. "It's really an unfortunate circumstance for the TWU."
Seniority agreements still need to be reached by the pilots and flight attendants unions.
Even if the leaders of United and Continental agree to merge their airlines, the hard work of combining two work forces with different unions and conflicting interests will remain.
The history of the airline industry is littered with cases in which peace in the boardroom was followed by rancor among co-workers at 30,000 feet.