The president of the nation's largest flight attendants union yesterday vowed that its members would go on a nationwide strike if US Airways and United Airlines are allowed to nullify their labor contracts in bankruptcy court.

Patricia A. Friend, president of the Association of Flight Attendants, said the group's leaders voted at the union's annual meeting in Pittsburgh yesterday to authorize a strike if the two airlines do not withdraw their motions to terminate their existing contracts and replace them with less expensive ones.

"We want them to withdraw their motions to terminate our contracts and agree to negotiate in good faith," Friend said. "But if that fails, on the day that the first flight attendant contract is thrown out by a bankruptcy court, we will exercise our right to self-help by withdrawing our services or going on strike."

Spokesmen for the airlines declined to say whether the carriers would withdraw their petitions to nullify the contracts.

US Airways spokesman David Castelveter said a strike would "ground this airline and send approximately 5,400 flight attendants to the unemployment lines. That option would not be in anyone's best interest."

United spokeswoman Jean Medina said the airline and the labor unions should continue negotiations to reduce the airline's costs. "This proposed action is not helpful to our employees or the success of our company going forward, or our industry," she said.

The union represents 46,000 flight attendants at 26 airlines, including two other carriers that are operating under bankruptcy protection: Indianapolis-based ATA Airlines and Hawaiian Airlines.

Flight attendants at carriers not represented by the union, such as American Airlines, Northwest Airlines and Continental Airlines, could be polled by their union officers to determine whether they would want to join in a job action. But their participation would be unlikely because they are bound by their existing contracts.

Any strike could face stiff obstacles. The union argues that nullification of the contracts in bankruptcy court would free the flight attendants from constraints imposed by the Railway Labor Act. Under the act, a labor action that disrupts the national transportation system must first be subjected to a federal mediator and a cooling-off period.

Washington-based airline attorney John J. Gallagher, who has done work for such carriers as Northwest, United and American, said the airlines would argue in federal court that even with a contract nullification a strike would violate the Railway Labor Act, possibly delaying any action for months. During the delay, however, the airlines could continue to lose money and may be forced to liquidate, Gallagher said.

Airline consultant Michael J. Boyd of Colorado-based Boyd Group, said calling for a strike would only "make the airlines weaker. "It's illegal and irresponsible and won't fix a damn thing."

The flight attendants' proposed action represents the most widespread response by a labor group in reaction to a series of industry-wide cuts in workers' pay and benefits. Friend said the union will mail out strike ballots to its members over the next two weeks. The union's goal, she said, is to have a vote count before the Christmas holidays.

"We've had enough," Friend said. "We're tired of playing the victim. We will not sit here quietly and continue to be abused."

Union President Patricia A. Friend, right, is cheered on by flight attendants as she announces the leaders' decision to authorize a strike.