What happens if the government shuts down? A lot, history tells us

The nation is barreling toward what could become one the largest government shutdowns in U.S. history beginning Oct. 1, with each of a dozen bills needed to keep funding flowing mired in Congress.

Medicare, Medicaid and Social Security payments won't be affected. Neither will the U.S. Postal Service, which uses its own revenue stream.

Still, union officials and other experts estimate the scope of this shutdown is on track to eclipse past spending lapses, with as many as 4 million workers affected -- about half of which are active-duty military and reservists.

According to the American Federation of Government Employees, roughly $5 billion a week in civilian workers' wages alone could get sucked out of the economy in a shutdown.

"It is uncharted territory. And it is incredibly stupid. I mean, it is the equivalent of burning down your own house," said Max Stier, president and CEO of the Partnership for Public Service, a nonpartisan organization that champions the value of government service.

Stier, AFGE and other experts say there is no telling how long this possible shutdown might last, either. President Joe Biden and House Republicans had agreed on a spending cap for the 2024 budget year earlier this year as part of a broader deal to prevent the U.S. from defaulting on its debt payments. But far-right House Republicans say they were never happy with that agreement and want to cut federal spending further.

Doreen Greenwald, head of the National Treasury Employees Union, said using government operations as political leverage doesn't make sense because of the devastating impact it has nationwide. Eighty-five percent of federal workers live outside of the Washington area and many of them live paycheck to paycheck.

In the last government shutdown, many workers were told to show up but couldn't afford gasoline for their cars, she said.

"You elect people to send them to Congress to make government run efficiently. This is not efficient," Greenwald said.

What happens next

Absent a deal on spending by the end of the day on Sept. 30, U.S. coffers would begin to dry up at midnight.

Many government employees would be told to report to work without pay, including service members and other "excepted" workers needed to tend to priorities like orbiting spacecraft, the power grid, federal prisons and airport security.

Contractors would be hit, too, including hourly workers such as janitors and security guards -- all while lawmakers on Capitol Hill would continue to get paid their $174,000-a-year salaries.

National parks would probably close, or at least their restroom and trash operations, leaving many of them vulnerable to vandalism. And some passport offices located inside federal buildings could close too, slowing down access.

According to the White House, an upcoming shutdown could delay new clinical trials for cancer and other research, halt food and environmental inspections, and put disaster relief programs at risk. It also could force 10,000 children to lose access to Head Start, a national early child development program.

Stier said that as disruptive as shutdowns are, many Americans might still be quick to dismiss the role government plays in their lives because "mandatory" programs like Medicare will continue and so many federal workers will be forced to show up without pay. But there will be lasting damage, he said, including attrition in the federal workforce.

"If someone says, yeah, these are just bureaucrats who are getting their comeuppance, I would say actually, these are people who are serving our veterans, who are finding criminals, who are keeping us safe, and helping Americans in all kinds of different ways ... There's no other class of Americans who are told you must work and you will not be paid at all (until) some future date," he said.

Likewise, Transportation Secretary Pete Buttigieg told Congress on Wednesday there could be ripple effects felt later on, saying it could make the nation's shortage of air traffic controllers even worse.

"We now have 2,600 air traffic controllers in training," he said at a House hearing. "A government shutdown would stop that training. Even a shutdown that lasts a week, two weeks, could set us back by months or more."

History repeating itself

The last time the government shut down- a history-making, 35-day fiasco over the 2018 holidays when then-President Donald Trump demanded a Democratic-controlled Congress pass funding for a border wall -- federal workers began showing up at food banks and many essential workers began to call in sick.

By the end of the shutdown on Jan. 25, 2019, sick calls by TSA workers resulted in long lines at airports across the country. Trump eventually relented without getting money for the wall. But $3 billion in U.S. economic activity evaporated, never to be recovered, according to the Congressional Budget Office.

If Congress doesn't act, this shutdown could be much bigger in scope. In 2018, Congress had enacted five of the needed spending bills. Currently, Congress has passed zero discretionary funding bills.

On the upside, all federal workers will automatically qualify for backpay once the shutdown ends thanks to legislation passed in 2019. Contractors though aren't necessarily so lucky, with pay decided by private employers who take a financial hit.

House Speaker Kevin McCarthy said Wednesday that he will still planning to get a deal.

"It's not September 30th. The game is not over. We are going to continue to work through it," he said.

ABC News' Arthur Jones and Amanda Maile contributed to this report.