The union that represents the nation’s air traffic controllers filed a lawsuit against President Trump and his administration on Friday in the latest effort to force the government to pay more than 14,000 people tasked with the high-stress job of directing air traffic as they missed their first paycheck after three weeks of working without wages.

The lawsuit, filed by the National Air Traffic Controllers Association in U.S. District Court in Washington D.C., comes as the shutdown takes its toll on airport workers, which also include Transportation Security Administration agents who screen passengers at airports.

Airport workers are among those considered critical to national security and who are mandated by law to continue working through the government shutdown.

In response, a number of security screeners have quit their jobs, while the number calling in sick has shot up, leading airports around the country shutting down security lanes and warning flyers to arrive early and expect delays.

Air traffic controllers have taken a different approach, so that "we're not seeing any unusual increase in sick leave," Doug Church, the deputy director of public affairs for the National Air Traffic Controllers Association, told the Daily Beast.

"But we have nearly 2,000 controllers who are eligible to retire and we are hearing that some want to retire now"—but because of the shutdown, "all relevant agencies are shut down."

So far, he said, there has been minimal impact in terms of missed shifts, "but we just missed our first paycheck. It would be irresponsible of me to speculate what it going to happen tomorrow.”

He said that air traffic controllers would begin a series of protests next week, starting as early as Monday, that will be "national in scale."

In Florida, officials at Miami International Airport issued a statement that they expect to shut down at least one concourse beginning Saturday through Monday and will transfer flights to other gates.

“In an effort to optimize resources without degrading screening and security effectiveness, TSA is working with key stakeholders and industry partners, and may explore efforts to consolidate officers and operations where feasible,” TSA spokesman Michael Bilello said in an email.

Airports began taking precautionary steps as the TSA said that more than 5 percent—or one in 20—of its 51,000 agents called out sick on Thursday. For the same day last year, 3.3 percent of agents were out sick, Bilello said.

Trump refused to sign a bill on Dec. 20 to keep the government open, setting a countdown to the partial shutdown affecting 800,000 federal employees.

Funding ran dry on Dec. 22, but Friday was the first time air traffic control agents, along with many other government employees, did not receive a paycheck.

According to Bloomberg News, this is at least the third time since the government shutdown began that the air traffic controllers union has sued over lack of wages. The union argues that the government violated worker's Fifth Amendment rights by unlawfully depriving them of earned wages without due process.

As The Daily Beast previously reported, this isn’t the only action airport workers are taking, as some TSA agents have already quit over their lack of pay.

One TSA agent, who has worked at the Philadelphia International Airport for eight years, told The Daily Beast he turned in his two-weeks notice on Friday, along with some of his co-workers.

“Four of my co-workers and I decided last week to quit today when we realized this shutdown was going to last a while,” the agent said. “I don't know what I am going to do now, but I can’t just sit around and wait while I’m not getting paid.”

The single father of one, who said the shutdown is extremely hard for him because he is a “one-paycheck household,” is now scrambling to find another job before his rent is due at the end of the month.

“I have no idea what I am going to do but at least now at least I am taking back control,” he said.

The toll the shutdown is taking on the airline industry was on display during a rally Thursday. Standing outside the Capitol, a large group of members from the National Air Traffic Controllers Association weathered the cold and wind to protest the shutdown. Many had received pay-stubs reflecting zero take-home pay just that day.

They were joined by members of a pilot union and a flight attendant union, who decried the risks to aviation safety they said the ongoing shutdown would create.

“No one needs to tell us what it means to not have the layers of security in place to protect the aviation industry, to protect the people who make the airplanes fly and to protect the people in our care on those planes and the people on the ground,” Sara Nelson, international president of the Association of Flight Attendants, said during her turn on the podium.

Nelson said when the layers of aviation security are not operating at their full capacity, “we are less safe and less secure.”

“Safety and security is non-negotiable,” she said, as those around her, many wearing purple NATCA golf shirts over their winter coats, cheered.

Speaking after the rally, several air traffic controllers described the hardship the uncertainty of not know when the next paycheck will come has put on their families.

“Trying to educate my kids is the hard part,” said Justin Sardineer, an air traffic controller from Cleveland Center, Ohio. He added that his wife was trying to work out an agreement with his daughter’s gymnastics school so she could stay enrolled while her family waits for the government to reopen.

Asked whether there had been talk among their colleagues of not showing up to work both Sardineer and Stephanie Ploenzke, also an air traffic controller from Cleveland Center, who was standing nearby, shook their heads.

“We are professionals, it’s our job to make sure people get safely from place to place,” Sardineer said.

“We fly,” Ploenzke said. “I would never want to put anyone else at a risk that I wouldn’t want to be on,”

“It’s our job, we swore an oath to keep everybody safe and that’s what we are going to keep doing” Sardineer said.

For married couple Kyle and Mary Hall, air traffic controllers from Shreveport, Louisiana, receiving the pay-stub with no salary made the shutdown all too real.

“We went from a two-income house to a zero-income house,” Kyle Hall said.

But even as the bills mount and they start to mull tough decisions, both Halls said they wouldn’t think of calling in sick.

“No, absolutely not,” the Halls said in unison.

“You need us there, at work,” Mary Hall said.

“The economy of the United State depends on air travel and air travel depends on us,” Kyle Hall added, “so we’ll be there, paid or not.”