The airline's flight attendants union issued a statement expressing "severe reservations" about a merger.
"United Airlines is run by the same management that took hundreds of millions of dollars in bonuses and stock for themselves paid with employee sacrifices of pay, pensions and health care," said Greg Davidowitch, head of the United branch of the Association of Flight Attendants. "There is no trust for these so-called leaders and no confidence in where they might lead us."
United's labor situation "is problematic," said Darrell Jenkins, a consultant to many airlines. "That's going to have to be handled."
Continental has done a better job of weathering the industry slump that began in early 2001 and was deepened by the terror attacks that year. Still, the company cut thousands of jobs, and just last year it pushed workers to take another $500 million in annual wage and benefit concessions.
While the industry is heavily unionized, there are pockets of nonunion workers. They are at most risk in a merger because a lack of representation, said James C. Little, international president of the Transport Workers Union.
In a novel organizing campaign in October, Little's union tried to convince Continental's baggage handlers and cargo agents that joining TWU would protect their jobs in a merger.
Rumors of a deal with United were already swirling. But the company downplayed the merger talk, and the workers voted narrowly to remain nonunion.
Little said the result might have been different if the election were held this week.
Associated Press Business Writer Dave Carpenter in Chicago contributed to this report.
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.
The most recent rumor involving the airline has it as a possible partner for Continental or Delta.
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General Vice President Robert Roach ,Jr. testified before Senate Commerce, Science and Transportation Committee on the potential impact of airline mergers and industry consolidation.