CHICAGO — A Teamsters union bid to win over 5,000 mechanics from a smaller union could play into United Airline's hands as it contemplates a merger and fields offers for its largest maintenance base in San Francisco.

The opening for the Teamsters occurred yesterday when he National Mediation Board called for an election between the Teamsters and the Aircraft Mechanics Fraternal Association, a small, independent union largely disowned by organized labor.

The wrinkle that plays to United's hands stems from its downsizing efforts.

Nearly half of United's 10,200 eligible voters were laid off by the airline years ago, and if neither the Teamsters nor AMFA win over 50 percent of the vote, that could leave a historically maverick group at United without any say in a merger.

It would also eliminate their veto power, written into their current contract, over any sale or divestiture of the San Francisco base, where more than 2,500 mechanics work. So, too, the mechanics would no longer be covered by a union, United officials added.

No date has been set for the election. But union officials said they expected balloting to begin next month and that voting would probably last three weeks to a month.

CONTINENTAL LINK

The Teamster bid comes amid widespread industry speculation of airline mergers with analysts pointing to a United-Continental linkup as most likely. Because the Teamsters already represent Continental's mechanics, a win at United could make it easier to meld worker groups if a merger of the two carriers goes through.

The roots for the showdown between the Teamsters and AMFA were sown nearly two years ago when disgruntled AMFA mechanics formed a dissident group. Friction intensified in December when the Teamsters filed a petition with the NMB to represent the mechanics.

Since then both unions have stepped up their campaigning, especially in San Francisco and in Indianapolis, where United closed a large maintenance base five years ago.

Besides putting 50 organizers into the fray and knocking on the doors of more than 4,000 United mechanics in recent weeks, the Teamsters' effort has been helped by a mechanic-organizer from the Transport Workers Union. The situation is unique in that the TWU also represents aircraft mechanics and belongs to the AFL-CIO, the labor federation that the Teamsters quit several years ago.

Teamsters officials say they decided some time ago that winning members among United's mechanics would help build its strength in the airline industry. The union already represents about 9,000 aircraft mechanics and has two other organizing drives under way besides the one at United.

MEMBERSHIP FALLING

The battle for the AMFA is about holding its ground at United, where it has represented workers since 2003, and across the industry.

AMFA has shrunk from nearly 20,000 members several years to about 11,000 members at seven airlines. One of the union's biggest challenges is overcoming the severe defeat it suffered two years ago at Northwest Airlines at the end of a 14-month strike. From 4,400 members before the strike, its ranks dwindled to several hundred members, said Steve McFarlane, AMFA's national director.

"We are going to aggressively defend our right to represent the (United) workers, and I'm confident that we will win," said McFarlane, adding that his group has received bids from disgruntled Continental mechanics who want to quit the Teamsters.

But Don Wolfel, president of AMFA's Local 4 at United, was less optimistic, saying he feared that neither union would win the vote, leaving the workers without representation.

"Our members really don't take an interest. They are fairly apathetic," he said.