WASHINGTON — Workers at U.S. airlines and railroads would have an easier time forming unions if the National Mediation Board succeeds in changing a 75-year-old rule on union organizing.

A proposed rule announced today would recognize a union if a majority of voting workers favor organizi

ng. Current rules require a majority of an entire work group to vote for a union in order for it to be certified. That means a worker choosing not to vote at all is effectively casting a “no” vote.

The issue lies at the center of a dispute at Delta Air Lines Inc. Unions representing flight attendants and ground workers who worked for Northwest Airlines before it was bought by Delta want the new rules to cover elections at the combined carrier.

“The current rules embrace a veto by silent principle that is not only unfair, it is undemocratic,” said Edward Wytkind, head of the AFL-CIO’s transportation trades department. “Just because a worker does not vote doesn’t mean he or she does not want a union, it just means he or she didn’t vote.”

That’s the rationale of the board, where two of its three members say current procedures are at odds with “the basic principles of democratic elections” and the idea of employee participation in workplace matters.

But NMB chairwoman Elizabeth Dougherty wrote a feisty dissent, questioning the board’s authority to modify a rule that goes back to the time of President Franklin D. Roosevelt. She argues that the current rules reflect the goal of keeping stability in the airline and railroad industries and avoiding any interruption in commerce.

Most large airlines, including Delta, also oppose the change. The Air Transport Association further claims it is unfair to change the rule on forming unions without also making it easier for airline workers to decertify a union.

“We agree with NMB chairwoman Dougherty that the proposal is a radical departure from long-standing and consistently applied rules,” said ATA president and CEO James May.

Delta spokeswoman Gina Laughlin said the proposed rulemaking “does nothing more than track the union’s talking points.”

“It is a stunning departure from the NMB’s historical process to objectively consider comments and seek a consensus,” Laughlin said.

The board will consider public comments for 60 days before considering whether to make the rule final.

The balance of power on the board changed earlier this year when President Barack Obama named Linda Puchala — the former head of a flight attendant union — to a seat on the board.

Union leaders say the rule change would place the airline and railroad industries under the same procedures as most other companies, which are overseen by the National Labor Relations Board.

The AFL-CIO’s Wytkind noted that nearly 100 percent of Delta flight attendants voted to unionize in 2008, but could not form a union because most other employees chose not to vote. Wytkind said future efforts to unionize workers would depend on a final rule change.