A 1981 White House memo outlined activities that would likely be exempted in a government shutdown, including medical care, air-traffic control, border protection and activities key to the safe use of food and drugs. Also exempted would be the essential elements of the banking system and tax collection activities.

All uniformed military personnel would be deemed essential and would remain on the job. Pentagon spokesman Col. David Lapan said the war in Afghanistan would continue. “You have to keep the Department of Defense operations going,” he said. “We have combat troops in the field.”

The administration could invoke the Feed and Forage Act, an authority that goes back to the Civil War and allows the military to incur basic expenditures, including pay and food.

The Postal Service wouldn’t interrupt service. “We’re self-funded,” said spokesman Gerald McKiernan. “Keep those cards and letters coming.” Federal law-enforcement agencies expect that many headquarters workers would stay home, while officers in the field would continue working. U.S. marshals who handle prisoners and fugitives would stay on the job.