If the government shutdown lasts through Sunday, it will be the longest shutdown in U.S. history. Among other things, that means hundreds of thousands of federal government employees will never have had to go this long without pay.

Most of the affected workers are taking it in stride so far. But cracks and signs of stress are already showing up. How long until they finally decide that enough is enough? And what happens when they do?

In some ways, this government shutdown is not as bad as some of its predecessors. Plenty of appropriations bills had actually already passed by the time the disagreement over Trump's proposed border wall reached its crescendo. As a result, only a fourth of the federal government's discretionary budget is actually impacted — and the discretionary budget is only about a third of the overall federal budget. The macroeconomic impact of the shutdown is likely to be negligible.

But that still leaves roughly 800,000 federal employees who are either furloughed without pay (about 380,000) or forced to work without pay because they provide "essential" services (about 420,000).

To understand how wild this is, imagine if a major private employer like Walmart or Amazon was able to legally force its employees to work for weeks without pay. It would be a massive scandal.

Historically, the legislative deals that finally ended shutdowns have also given workers their back pay. But this is not true of people employed by private contractors who then work for the federal government — a description that applies to a growing share of the federal workforce, most of them low-wage already.

Beyond that, the vast majority of federal workers are not so comfortable as to be able to go weeks without a paycheck. A little over 12 percent of U.S. federal workers earn at most $40,000 a year. "Right now, I'm worried about whether or not I can make my credit card payment, whether I can pay for the electricity, get any food, pay the rent, whether or not I'm going to be thrown out onto the street," Paul Kiefer, one of the $40,000-or-less IRS employees, told NPR. "It's that serious."

About 4 percent of federal workers earn over $140,000 a year. The rest fall somewhere in the mushy middle. Yet the rise of inequality, and the vertiginous cost increases in basic needs like housing, health care, and education, mean even many Americans making six figures more or less live paycheck to paycheck. If that flow of income suddenly stops, things can get dicey pretty quick.

"We are a paycheck to paycheck family," Brian Turner, a Philadelphia TSA officer, told NBC. "We have a mortgage payment. We have credit card payments. We have car payments. Utilities. And we also have child care. So with half of our income gone, it is very concerning."

With President Trump still not budging over his demand for $5 billion to fund a border wall, there's no sign of when the shutdown will end. What happens when the strain on federal workers becomes too much?

The first option is they could start quitting at much higher rates and look for work elsewhere. That's certainly not a guarantee of relief. But with unemployment at 3.7 percent, the private market is better positioned to soak up an exodus from the federal workforce than it has been in a very long time. "We can handle a month or two, but if it gets much longer than that, I'm going to look for another job — a job in the private sector," Melissa Sims, a prison nurse and one of those "essential" federal employees, told The New York Times.

Federal workers could also sue the U.S. government for violating the Fair Labor Standards Act. In fact, the American Federation of Government Employees (AFGE), the country's biggest federal employee union, did just that at the start of January — the plaintiffs, again, are "essential" employees who must work without pay. The same law firm sued the U.S. government over the 2013 shutdown and won. The court ordered the government to pay federal workers double what they were owed. (Though about 25,000 of those workers still haven't gotten the full payment.)

If the shutdown drags on, the Trump administration could very well face more such legal action. "Most of our members' take-home pay is $500 a week," David Cox, the president of the AFGE explained to PBS. "That is not a lot of money."

Finally, federal workers could try going on strike.

This is dicey, since it's technically illegal for federal workers to do so. That doesn't mean they can't strike, but it does mean they'd be getting themselves into an extremely dangerous political fight they'd better win. The last time this happened was the federal air traffic controllers strike in 1981 — which they lost, breaking the union and ruining many of the workers in the process. "History has shown that illegal strikes in themselves are not what is truly dangerous for workers," Jacobin argued recently. "What is dangerous is unsuccessful strikes. Half-hearted organizing might be more dangerous than no organizing at all."

It'll probably take a lot more pain before federal workers would contemplate such drastic action. But there are already signs of strain: Rates of TSA screeners at major airports calling in sick have ticked up under the shutdown. The TSA itself insist the situation is totally manageable, but observers are getting nervous.

President Trump, in his usual Trump-ian fashion, has been completely tone deaf on the subject. He's has swung from claiming the federal workers not being paid are Democrats (thus implying they somehow don't count) to claiming they mostly support him. Trump's Office of Personnel Management even suggested federal workers offer to paint or do carpentry for their landlords to pay the rent.

The situation isn't critical yet. But with every paycheck that's missed, more gunpowder will be added to the keg. Eventually, it may well blow up in Trump's face.