The Obama administration may have given America’s labor unions a big break in their attempt to organize flight attendants, including about 20.000 at Delta, now the world’s largest airline and, historically, the least unionized of major U.S. carriers.

What’s changed is a 75-year-old rule that required a “yes” vote from the majority of an airline or railroad’s entire work force within a “class” or “craft” (such as pilots or flight attendants) in order to unionize. On Monday, the National Mediation Board announced new rules that would only require “yes” votes from the majority of employees who actually cast ballots.

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“This rule change… is arguably the most controversial change that the National Mediation Board has ever undertaken, at least in my 30 years of watching the industry,” said William “Bill” Swelbar, a research engineer with the International Center for Air Transportation at the Massachusetts Institute of Technology. “It appears quite driven by an administration’s policy to promote unionization and collective bargaining.”

Supporters say the new union voting procedures for airlines and railroads are no different from those long in place at other industries and, for that matter, democratic elections in general.

“The National Mediation Board (NMB) has taken an important and essential step towards ensuring a more democratic process for airline and rail workers seeking representation in the workplace,” AFL-CIO President Richard Trumka said in a press release following the board’s decision.

But opponents fear the new rule could spark labor disputes at a time airlines are struggling to recover from the recession, increased fuel costs and security restrictions.

The Air Transport Association, a trade organization representing major U.S. carriers, issued this statement:

“We continue to believe the National Mediation Board does not have legal authority to implement this rule, one that undoubtedly will lead to more labor discord. It is quite clear to us that the NMB was determined to proceed despite the proposed rule’s substantive and procedural flaws, leaving us no choice but to seek judicial review.”

Delta issued this written statement:

“While disappointed, we are not surprised by the majority members’ decision in view of the way this rule change has been handled. Given the level of opposition to this change and the process that was used, we expect there will be legal challenges. Delta will join in support of a lawsuit when it is filed.”

According to figures compiled by The Associated Press, roughly two-thirds of airline and railroad workers already belong to unions — compared to the nation’s overall union membership rate of 12 percent.

Delta was an exception, with only 15 percent union membership, before its recent merger with heavily unionized Northwest Airlines.

“Delta has created a culture,” Swelbar said. “It has paid its people very well over time. And in return for that good pay, Delta has a highly productive workforce. But, obviously, they’ve created a culture over their history where employees did not see the need for unionization to advance their career.”

Unions failed to organize Delta flight attendants two times under the old rules.

As for the impact the new rules will have, Swelbar said, “I don’t know that it’s a done deal. But let’s just say that this rule change makes unionization much easier. And so if that is the case, then, yes, I expect that Delta will see more unionization tomorrow than it has today.”