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Just who exactly is considered 'essential'?

From NBC's Pete Williams
Federal law prohibits government employees from coming to work during the shutdown except for those needed in "emergencies involving the safety of human life or the protection of property."

So how does that amount to the roughly 60 percent of the federal workforce?

A 127-year-old law, the Antideficiency Act, bans the government from spending money, or incurring debts, that Congress hasn't approved.  That means in a shutdown, federal employees cannot come to work and earn a salary, for which they would have to be paid.

Congress amended that law in 1990 to specify which federal employees would be granted exceptions and required to continue working to cover emergencies. The  new law says emergencies worthy of an exception do not include "ongoing, regular functions of government, the suspension of which would not imminently threaten the safety of human life or the protection of property."

A 1995 internal Justice Department legal memo said the law must be interpreted narrowly -- "only when a threat to life or property is imminent" and "of such a nature that immediate action is a necessary response to the situation."

It's an understatement to say the law has been read generously. "It's more a matter of custom than strict interpretation," says a former Justice Department official.

Broad exceptions are made for virtually the entire federal law enforcement, including the FBI, DEA, ATF, US Marshals, ICE, and CPB.  Roughly 80% of employees in the Department of Homeland Security will stay on the job, including the Coast Guard and the Secret Service.  Federal meat and poultry inspectors will report for duty as usual.

A White House official says the fact that more than half of all federal employees are considered exceptions is driven by "an increase in civilian DoD and DHS personnel, as well as new sources of funding outside of appropriations -- most notable, the Veterans Administration now gets advanced appropriations, so they stay open."

The Social Security Administration is also funded by an indefinite appropriation, so many of its employees will be expected to remain on the job.

Still, the fact that roughly 1.2 million of the nation's 2 million federal employees will be on the job after the shutdown seems a departure from what the Justice Department said in 1995, when it advised the White House that the emergency exception "applies only to cases of threat to human life or property where the threat can be reasonably said to be near at hand and demanding of an immediate response."