Meanwhile, the Association of Flight Attendants, which represents 21,000 flight attendants at the two carriers, says it will oppose a merger unless salaries and working conditions are improved first. And the International Association of Machinists, the largest airline union, flatly opposes a deal. The IAM represents 3,400 fleet service workers and 3,000 mechanics at US Airways, as well as 16,000 agents and fleet service workers at United.

As for United's 9,300 mechanics, they voted last month to join the Teamsters, so a merger could lead to a representation battle with the IAM. The Teamsters will oppose any deal that is "to the detriment of our members," said spokesman Galen Munro.

Congress has no direct influence over whether mergers occur, but it has taken an interest in the union representation issues in the Delta-Northwest merger, and no doubt it would weigh in on a United-US Airways deal as well.

Last month, in a Senate Judiciary Committee hearing, New York Sen. Chuck Schumer called the Delta and Northwest merger "a good one" and said the "negatives seem more benign" than in other transactions. Still, he said that "anyone that supports this merger does not indicate a general support of mergers in the airline industry."

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