They call it a "government shutdown." But of about 4.1 million people who work for the federal government, about 80% will still be expected to show up for work.
We still don't have an exact number of federal employees who won't be working in a shutdown, but most press reports have been pegging the number around 800,000, the number who stayed home the last time the government shut down in 1996.
The roughly 3.3 million federal employees who will have to work fall into three main categories:
1. Workers whose pay is not subject to annual appropriation. Remember, a shutdown only means there's no money for "discretionary spending," the part of the federal budget that must be appropriated annually by Congress. But a lot of federal employees are paid from sources other than appropriations.
The biggest example here is the U.S. Postal Service, which has over 500,000 employees and which is (more or less) funded by its own postage revenues. Other examples include the U.S. Mint (funded by the sale of coins), the Federal Reserve (funded by profits on its investment activities) and the Office of the Comptroller of the Currency (funded by fees on banks).
Some government agencies that are funded only partly by appropriations will be able to stay open (and pay their employees) for a while during a shutdown even if they couldn't stay open without congressional appropriations forever. The State Department will continue consular operations, such as the issuance of passports, "as long as there are sufficient fees to support operations." Amtrak expects to stay open because it can rely, at least for a while, on ticket revenue.
These workers are the lucky ones: Not only are their jobs not disrupted by a government shutdown, but they'll also get to collect paychecks while the government is shut down.
2. Workers whose jobs are necessary to carry out activities that are not subject to annual appropriation. Here's the best example: Social Security benefits are mandatory spending, meaning they're supposed to go out even if Congress has not passed any appropriations bills. But the workers who actually process Social Security benefits have their salaries paid out of appropriations.
In general, the Antideficiency Act prevents government agencies from agreeing to pay amounts that Congress hasn't appropriated. But presidents including Reagan and Clinton have relied on a legal opinion saying that mandatory spending programs like Social Security have a "necessary implication" that you can keep necessary employees around to ensure mandatory funds get spent, even if those employees' own salaries are not covered by mandatory spending.
In other words, the people sending out Social Security benefits have to keep working when the government is shut down. But that doesn't mean they can be paid: Their paychecks will be held up until Congress appropriates money to make the payments.
This is also why Obamacare implementation will continue even when the government shuts down. Benefits under Obamacare are mandatory spending. And the administration will be able to keep workers around to implement those benefits, even if those workers' salaries are supposed to be appropriated.
3. Workers whose jobs are essential to protect life or property. This isn't a loophole the president can use to keep anyone on the job by saying his or her work is "essential." The exception applies only "to cases of threat to human life or property where the threat can be reasonably said to the near at hand and demanding of immediate response," according to that same legal opinion.
The Washington Post has a roundup of agencies with many employees deemed "essential" under this exception. Agencies where most employees will still be working include the Department of Homeland Security (including the TSA, Secret Service, Customs and Border Patrol, and FEMA), the Department of Transportation (including the FAA), the Department of Veterans' Affairs (including VA Hospitals) and the Department of Justice (including the FBI, DEA, and Bureau of Prisons).
Active duty military, all 1.4 million of them, will stay on the job, as will about half the Department of Defense's 800,000 civilian employees. Most "essential" civilian employees will have to wait for the government to reopen before they can collect their paychecks, but on Monday President Obama signed a bill ensuring that active-duty military will be paid even if the government is closed.