ATLANTA ( TheStreet) -- Delta's ( DAL) long-standing, largely union-free status is up in the air as the National Mediation Board engages in an unusually contentious debate over changing a rule that could make it easier for airline workers to unionize. Delta, the world's largest airline, acquired heavily unionized Northwest in 2008 and now is headed toward a series of representation elections. Two of the biggest airline unions, the International Association of Machinists and the Association of Flight Attendants, are battling to retain their standing at the merged carrier. Delta employee costs would likely rise in any case. Either the carrier will bring Northwest employees, who signed contracts in bankruptcy, to Delta levels, or it will negotiate new contracts with unions. But Delta has said repeatedly that it prefers to deal directly with employees, without a third party involved. A few months after Delta completed the Northwest acquisition, Barack Obama's election brought a new composition to the NMB, which oversees Railway Labor Act implementation at transportation companies. Now the board is seeking to change a long-standing provision of the nation's labor laws. Currently, unions face an unusual burden in RLA elections: They must win support from a majority of the people eligible to vote, not just a majority of the people who do vote. The law has been in place for 75 years, under the theory that labor strife ought not to disrupt essential transportation, and generally has not prevented unionization in the airline industry.