Growing lines at airports. Transportation Security Administration agents not showing up to work. The closing of a terminal in at least one major airport — Miami International. Air traffic controllers passing out leaflets to airline passengers.

The partial federal government shutdown already has lasted four weeks, and President Donald Trump’s proposed deal Saturday to end the impasse was drawing swift criticism from Democrats.

If the shutdown continues, could a growing crisis at the nation’s airports become the impetus for ending the federal furloughs?

“It’s a fair question whether this might be the kind of thing that might apply pressure to end the shutdown," said Seth Kaplan, editor of the commercial aviation publication Airline Weekly. But, “it hasn’t worked yet.”

“I think it’s early to say whether that will actually transpire that way," he said. "But it’s reasonable to think that it could.”

A relatively small percentage of airline passengers have experienced delays at airports related to the shutdown, the longest in U.S. history. It hasn’t affected all airports, and this is a quiet time for the airline industry, Kaplan said. But if the shutdown continues past mid-February, when demand picks up for spring-break trips and travel for Passover and Easter, the shutdown could become a bigger problem, he said.

The airline industry differs from many others because of how “visual” it is, and because when something goes wrong at an airport, people experience it communally, he said.

The partial government shutdown began Dec. 22 after Democrats refused to agree to give more than $5 billion for the wall Trump wants to build along the U.S.-Mexico border.

It continues to affect varied aspects of people’s lives — those who provide government services and those who depend on them.

Some federal workers and contractors who are not getting paid and may be living paycheck to paycheck are struggling. Food agencies are setting up an “emergency market” in Philadelphia for federal employees. Nearly two million Pennsylvanians received food assistance benefits Wednesday for February — weeks early, and people who don’t budget accordingly could run out of food. The U.S. Department of Agriculture has not said what will happen to the benefits in March.

Federal courts are running out of money. Immigration court cases are piling up. National parks, including Independence National Historical Park, are closed. Furloughs could cut into income tax revenue. Economic growth slows with every week the shutdown continues, the White House has said.

Will those impacts, individually or collectively, affect the duration of the shutdown?

“The kinds of pressure points we might normally be thinking about might not apply,” said Katharine Young, an associate professor at Boston College Law School who studies comparative constitutional law and has written about government shutdowns. “The playbook that Trump is using is one that is really thriving in the shutdown and the destabilization that it’s producing. We have to deal with the worry that this is potentially almost endless.”

But long lines at the nation’s airports ultimately could make a difference, Kaplan said.

“You can imagine [airport delays] being the kind of thing that would perhaps have a disproportionate impact compared to other things that are really important too but just aren’t as visual as people being stuck in long lines at the airport," he said.

The TSA said Friday it expected a higher volume of travelers over the Martin Luther King Jr. holiday — more than eight million passengers, nearly 11 percent more than on the same long weekend last year. Meanwhile, the national rate of unscheduled employee absences is 6.4 percent, compared to 3.8 percent in the same period last year.

Hundreds of thousands of federal workers — including TSA agents — did not get their most recent paychecks Jan. 11.

The president has ordered thousands of federal employees to go back to work without pay to soften the effects of the shutdown.

But some of the more than 50,000 TSA employees nationwide evidently are quitting to seek other jobs.

Given the issues with TSA staffing, and the increase in holiday travel, Douglas Kidd, executive director of the National Association of Airline Passengers, said passengers should get to airports earlier and pack light.

“We are more than a little put out with all the parties involved in this," Kidd said. “If you can avoid traveling by air, maybe now is a good time to avoid traveling by air."

Ironically, one thing that has allowed the shutdown to go on is the fact that some TSA agents and other federal employees are working, said Dan Sobien, a meteorologist who is president of the National Weather Service Employees Organization. That’s the union that represents government forecasters, for whom this will not be a holiday weekend.

“If the government truly shut down,” he said, “there wouldn’t be any more government shutdowns.”