Family feuds die hard and a perennial one over unions at Delta Air Lines is getting a new and highly visible airing.

An organizing effort by the airline’s ramp workers and flight attendants took a turn in the spotlight of viral social media earlier this month. As a union enlists backing for a vote, the airline is fighting back with tactics that some union leaders think is not fair and may be illegal.

To dissuade support, Delta placed posters in breakrooms that encouraged employees to spend money on beer and video games rather than union dues. Workers took photos of the posters and then posted them on social media. Others passed the photos along and, soon, the workers were receiving support from politicians and people who didn’t know about the clash.

“Delta is helping us with some of their tactics that are really enraging,” said John Coveny, national campaign coordinator for the International Association of Machinists and Aerospace Workers (IAM). “We’ve had an increase of signatures from people who have never signed cards.”

The dispute shows Delta hasn’t fully overcome the labor legacy of Northwest Airlines, the Minnesota-based carrier it acquired 11 years ago. Worker support for the union is strongest in hubs like Minneapolis-St. Paul and Detroit that it inherited from Northwest.

Leaders at the Atlanta-based airline refer to its 80,000-person workforce as the “Delta family.” But they have long been resistant to unions.

When Delta acquired Northwest, the different work groups voted for or against unionization. Today, Delta’s pilots and dispatchers are the only unionized groups. For years, factions within the flight attendant and ramp groups have unsuccessfully tried to unionize. Delta said it generously compensates workers and that the organizing effort by the flight attendants and ramp workers is unnecessary

A group of U.S. senators, led by Bernie Sanders, the Vermont independent who is also running as a Democratic presidential hopeful, disapproved of the airline’s anti-union actions in a recent letter to Delta Chief Executive Ed Bastian.

Bastian responded to the allegations with his own letter. He said the posters were in poor taste and assured Sanders that they had long ago been removed, but he defended the company’s position and accused the senator of misrepresenting the facts.

“Delta is and always will be pro-people. That commitment shows in our track record,” Bastian said in the letter, which also had a list of company benefits to workers.

Within days of the heightened publicity, the IAM filed an election-interference complaint against Delta with the National Mediation Board, claiming company managers are using intimidation and coercive tactics, including unfairly disciplining union backers, to suppress support.

And while the posters got much of the attention, they aren’t illegal, said John Budd, a labor-relations expert at the University of Minnesota.

“The accusations against the company are really much more serious than those posters,” Budd said, “and I’m worried people will conflate the two.”

The line between legal and illegal campaign tactics is blurred, he said. Sharing facts, opinions and personal experiences is permissible whereas threats, intimidation, promises and spying are not allowed. Delta maintains that its actions are fully legal and that it respects its employees’ right to organize, but that it simply wants to share the facts of unionization.

“The problem is there are a lot of gray areas and questions of when facts and opinions bleed over and are interpreted as a threat,” Budd said.

The National Mediation Board governs labor disputes in the airline industry under the Railway Labor Act and will have to decide if Delta’s campaign crossed a legal line.

“If those allegations were true, it would be enough to show the election had been interfered with,” said Gregg Corwin, a Minneapolis-based airline labor-relations lawyer. “But there’s a difference between an allegation and the ability to prove it, and that’s the hard part.”

These investigations are a legal process that can take months or years and often can pit one person’s testimony vs. another’s version of the same conversation.

“Just delaying is a tactic that companies use because a delay typically favors the employer,” Budd said.

The groups can’t unionize without an election, and an election won’t be held unless more than 50% of the workers sign an authorization card. Those signatures are only valid for a year, so the union is constantly returning to members and asking them to sign a new card, which are then valid for another year.

Delta employs about 16,000 ramp and fleet-services agents. The union currently has more than 9,000 signed cards on file, but some of them are expired, according to the IAM.

“We have to run through 45 different cities to get signatures,” Coveny said. “As cards fall off, we not only have to get new cards, we have to chase old cards.”

Apart from former Northwest fortresses of Detroit and the Twin Cities, the union is seeing strong support in Delta’s New York, Los Angeles and Las Vegas hubs. Workers in Seattle, Salt Lake City and Atlanta appear less supportive.

Union membership has been in steady decline for decades. Last year, just 10.5% of U.S. workers belonged to a union, down from more than 20% in 1983 when the Bureau of Labor Statistics started tracking the data.

Unlike the National Labor Relations Act where workers can organize at the local level, the Railway Labor Act governing airlines requires that the union decision is made systemwide for an entire class of workers, Budd said.

“This just adds to the logistical challenges for the union. You have different cultures in different areas with some being more pro-union or anti-union,” Budd said. “It also ramps up the intensity on both sides because it is all or nothing.”