Federal agencies have some latitude to decide what passes muster, though that discretion comes with risk for workers on the front lines. Federal employees who “knowingly and willfully” violate the law face fines of as much as $5,000 and up to two years in prison.

The Government Accountability Office regularly tallies up possible offenses, but it’s up to the Justice Department to prosecute. No one has ever been charged with violating the Antideficiency Act.

And because the issue has never really been litigated, the ground rules aren’t clear. Agencies are left trying to discern legal opinions from the Justice Department and guidance from the Office of Management and Budget.

“The administration can exercise a fair amount of discretion in deciding what counts as excepted workers and what counts as protection of life and property,” said Georgetown University law professor Timothy Westmoreland.

Although government lawyers have spent decades writing guidance, “if someone now comes into OMB and DOJ and decides simply ‘we’re going to ignore that and order something different,’ I’m not so sure what can be done about it,” Westmoreland said.

The National Treasury Employees Union thinks it has an answer. It has gone to court, arguing that forcing certain employees to work during the shutdown is unconstitutional. “The Constitution commands that ‘No money shall be drawn from the Treasury, but in Consequence of Appropriations made by Law,”’ the union says in the lawsuit, which was filed in federal district court in Washington and seeks an order to keep the government from forcing NTEU members to work during the funding lapse.

A case can be made that some federal employees are protecting human life and property, said NTEU president Tony Reardon. But “that line of reasoning gets quite shaky when applied to thousands of IRS employees being called back in order to process tax refunds -- and to do so without being paid.”

Uncharted Territory

The case shows how the U.S. is entering uncharted territory as the government shutdown enters its fourth week.

Real-world dilemmas may soon compel answers to previously academic questions about Congress’ power of the purse and the Antideficiency Act. Many federal employees are set to miss their first paycheck Friday.