Federal agencies are scrambling to activate contingency plans for who would stay home and who would come to work this weekend if the government shuts down because of Congress’s failure to clear a spending bill by midnight Friday.

With Senate Democrats threatening to stall a short-term spending bill as they push for more-generous health benefits for retired miners, it looked more likely Friday afternoon that government operations could stop at midnight. A shutdown could last as little as a few hours into Saturday morning or until Sunday afternoon.

Even a short lapse would have some effect on the public.

National parks and public lands would close, as would the National Zoo, although the animals would still be fed. The Smithsonian, another federal institution that’s open to the public on weekends, would close its doors.

Park service officials spent Friday discussing whether they would need to rope off the World War II Memorial on the National Mall and other monuments.  But more than 400 parks, national monuments and historic sites across the country would have to close, because there would be no staff to manage them.

The government would not stop functioning, of course: Air traffic controllers and airport security screeners would stay on the job, the borders would be patrolled and military operations would continue.

Federal prisons and veterans hospitals would stay open and the Forest Service would keep fighting fires because those are essential operations for health, safety and national security. And government operations not directly paid for by the Treasury Department, the largest of which is the U.S. Postal Service, also would continue. Those employees would be ordered to come to work.

Many offices and institutions that normally would close in a government shutdown because they are considered “nonessential” would not be affected, because they are closed on weekends.

Lawmakers are considering a stopgap spending bill that would keep federal agencies funded through April 28. Senate Democrats, led by Joe Manchin III (W.Va.), are threatening to force a brief shutdown to pressure Republicans to support policies they say President-elect Donald Trump endorsed on the campaign trail, in this case to help coal country and a separate provision that would require U.S. steel to be used in some future infrastructure projects.

The angry Democrats are threatening not to block the spending bill but to delay its passage past the Friday deadline in hopes they can entice Republicans into further negotiations for an extension for the miners’ health benefits.

The White House Office of Management and Budget, which is responsible for communicating with federal offices when Congress cannot agree on a budget, said it is in touch by phone and email with heads of agencies, asking them to dust off contingency plans they last used during a two-week partial shutdown in 2013.

“Prudent management requires that the government plan for the possibility of a lapse and OMB is working with agencies to take appropriate action,”  agency spokeswoman Emily Cain said in an email.

But the White House is waiting to pull the trigger on setting in motion a government closure until Friday evening.

“It is our hope that this work will ultimately be unnecessary and that there will be no lapse in appropriations,” Cain said. “There is enough time for Congress to prevent a lapse in appropriations by passing a short continuing resolution so they can get back to work after the holidays and actually pass a budget through the remainder of the fiscal year.”

Of the 13 full or partial government shutdowns since 1980, four have been for only one day, two others for two days and three others for three days. Since they were so short and the government was confident that funding for operations would flow quickly, there was little disruption to government services.