Northwest Airlines' largest labor union says it is prepared for a vote to represent nearly 13,000 ground workers at Delta Air Lines. The International Association of Machinists is asking the National Mediation Board to declare that the two airlines are acting as a single carrier, paving the way for a vote on representation. The vote will be restricted to luggage and cargo handlers, flight simulator technicians and security workers at Delta, although the union also represents thousands of customer service agents and similar positions at Delta. "There's obviously some question as to whether they would get the vote they want with everyone voting," said industry expert Bill Swelbar of the union's strategy, noting that a partial vote "could at least allow them to get a foot in the door at Delta." A Delta executive said the company was "disappointed and quite puzzled" at the union's move to bar some 20,000 workers from the voting process.