The Delta Pilots Association (DPA) claims that nearly half of the airline’s 12,000 pilots have at some time in the past four years signed cards backing it to be their union for collective bargaining with management. But under U.S. labor law, many of the cards – a third or more – are invalid because they weren’t signed in the past 12 months, DPA said.

    The group is trying to oust the Air Line Pilots Association (ALPA), which has represented pilots at the carrier since 1940. To take ALPA’s place, DPA would need more than 50 percent of the pilot workforce to sign valid cards within a 12-month period to trigger an election through the National Mediation Board, during which all of Delta's pilots would vote to determine which union should represent them.

"The longer time goes, the harder it seems to become," said Amie Howard, a first officer who volunteers for DPA. "I think we need to get a little more creative... We've got a contract coming up. Now's (the) perfect time."

DPA says the number of people it has signed up has slowed since 2012 contract talks. Negotiations for the next contract can begin in April 2015.

"We're kind of fighting apathy," said Tim Caplinger, who founded DPA in 2010 and is also a first officer. Still, he said he was hopeful of making a breakthrough in the coming months.

     ALPA, which also represents United Airlines pilots and 28 smaller pilot groups, says that its size gives it wide impact and that few pay attention to DPA.

     Delta declined to comment. 

DPA organizers say they will push Delta more aggressively to share recent financial gains with workers and have accused ALPA of being cozy with Delta management, a charge ALPA rejects. 

    "When our interests are aligned (with Delta's), we link our arms with our management," said Captain Mike Donatelli, the leading Delta representative in ALPA. "When our interests are not aligned, then we will go our separate ways."

Delta filed for bankruptcy in 2005, which led to pay cuts and layoffs throughout its workforce. It has recovered since and its net profit climbed to $10.54 billion in 2013, buoyed by a merger with Northwest Airlines, and as the airline industry has avoided adding much capacity. More recently a big decline in oil prices will be helping its financial performance.

According to Caplinger, the biggest challenge remains reaching more pilots, many of whom Delta recently hired. Delta prohibits DPA from signing up members in its pilot lounges, one of the prime areas for pilot contact.

(Reporting By Jeffrey Dastin in New York and Saundra Amrhein in Tampa; Editing by Alwyn Scott and Martin Howell)

By Jeffrey Dastin and Saundra Amrhein