"We don't have a plan yet," said Abby Wines, spokeswoman for Death Valley National Park in California, which is seeing 80,000 visitors a month. "We just got a memo about this yesterday. Today's the first day we're seriously thinking about this."
Other federal agencies were preparing plans to close. Across Washington, senior officials were pulling out the detailed manuals that tell them whom to send home and whom to keep on the job if the government runs out of money, either because their mission is essential or is funded by a source other than Congress. About 800,000 federal workers were furloughed during the last shutdown in 2013, which lasted 16 days. Roughly the same number would be affected today.
The full impact of a shutdown would not be apparent until Monday. In the District, the Smithsonian's 17 museums and the National Zoo would remain open Saturday and Sunday, using funds from previous appropriations. They would close on Monday — though the zoo animals would still receive food and care. The National Gallery of Art also is planning to stay open this weekend to accommodate crowds flocking to its popular "Vermeer and the Masters of Genre Painting: Inspiration and Rivalry" exhibit, set to close Sunday.
Monday also would bring the cessation of some other services: Some long-term recovery efforts would be affected in areas stricken by catastrophic hurricanes, wildfires or mudslides, officials said. The Internal Revenue Service would send home thousands of employees, leaving questions about the new tax law unanswered.
The Centers for Disease Control and Prevention would furlough about 61 percent of its staff — roughly 8,400 people — in the midst of one of the harshest flu seasons in recent years. And the Education Department would furlough 90 percent of its staff in the first week — though employees serving 13 million students who use financial aid to pay tuition bills would stay on the job.
The most crucial government functions would be performed throughout any shutdown: Air traffic controllers and airport security screeners would come to work; the borders would be patrolled; military operations would continue. Federal prisons and veterans hospitals — the only agencies for which Congress has approved funding — would also stay open. The Agriculture Department would continue inspections of meat, poultry and eggs, and the Forest Service would keep fighting fires.
The U.S. Postal Service — which is not directly funded by Congress — would continue to deliver the mail. And special counsel Robert S. Mueller III would continue his probe into Russian meddling in the 2016 election uninterrupted, since it is funded with a permanent, indefinite, appropriation.
The shuttering of iconic parks proved to be a political flash point during government shutdowns in 1995 and 2013. On both occasions, Republicans controlled Congress and a Democratic president sat in the White House; both times, Republicans shouldered much of the blame for ruining people's vacations.
This time, White House budget director Mick Mulvaney proposed keeping the parks open in the event of a budget impasse, according to an administration official who spoke on the condition of anonymity to discuss internal deliberations. That triggered discussions between top officials at the Interior Department and the Park Service and administration lawyers to determine whether and how to preserve public access to national parks.
In an interview, Sen. Michael F. Bennet (D-Colo.) described the effort as a calculated ploy to obscure the real impact of failing to reach a spending deal.
"It's purely motivated by politics," Bennet said. "But no matter what they do, there will be no way to cover up the many catastrophic effects of a shutdown."
President Trump and congressional GOP leaders, however, argued that Democrats were spoiling for a shutdown fight because it would distract the public from the recently passed tax bill.
"I really believe the Democrats want a shutdown to get off the subject of the tax cuts," Trump told reporters ahead of a campaign stop in Pennsylvania. "That is not a good subject for them, the tax cuts."
With Republicans now in control of the White House and both chambers of Congress, many fear they would shoulder the blame for failure to strike a deal to keep the government open.
Mulvaney pledged in an interview on Fox Business this week that a shutdown "would look very different under a Republican administration than it would under a Democrat." He cited national monuments, which he said would stay open.
Late Thursday, as lawmakers labored to forge an agreement to avert a shutdown, administration officials said they were laying plans to keep many parks open for hiking, wildlife watching, snowshoeing and cross-country skiing.
Depending on the location, park concessions — hotels, gas stations, gift shops and food stores run by private companies — might stay open as well, officials said.
"We fully expect the government to remain open. However, in the event of a shutdown, National Parks and other public lands will remain as accessible as possible while still following all applicable laws and procedures," Interior spokeswoman Heather Swift said in an email. "Visitors who come to our nation's capital will find war memorials and open-air parks open to the public."
The department "will still allow limited access wherever possible" to national parks, refuges and other public lands, Swift added, including on roads that have been cleared of snow. "Wilderness type restrooms . . . will remain open," too, she wrote. But "services that require staffing and maintenance such as campgrounds, full service restrooms, and concessions will not be operating."
Open gates at national parks and monuments would stand in stark contrast to the last government shutdown, in 2013. The Obama administration erected barricades around popular sites to mark the closures, which quickly became potent symbols of government dysfunction. In Washington, busloads of elderly veterans, many of them in wheelchairs, angrily pushed aside barricades to tour the World War II Memorial on the Mall.
Late Wednesday, Lena McDowall, Interior's deputy director for management and administration, sent park officials a "draft contingency plan" outlining how they could stay open.
But she cautioned that the plan was still in flux: "The Solicitor's Office is still reviewing and continues to make additional suggestions and edits, so this is probably not the last version you will see."
Experts on the national park system said providing access when the parks are not adequately staffed could pose serious risks to tourists as well as to the parks themselves. Park staff provides safety guidance to visitors, including which trails are safe and what sort of equipment is needed to traverse them.
Theresa Pierno, president of the National Parks Conservation Association, said in an email that "the vague direction" superintendents are receiving from headquarters puts them "in an impossible situation."
"They'll be forced to make on-the-fly decisions about what areas of national parks warrant protection. And then they'll need to determine how to protect those places with virtually no staff," she said, adding that the policy "raises some serious questions not only about what resources get protected, but also the legality of this partial closure scenario."
John Czwartacki, a spokesman for the White House budget office, said it makes sense that the administration is not going to use shuttered parks as political ammunition the way Obama did five years ago.
"There is no desire to weaponize closing of public parks or monuments for partisan, political reasons," he said.
In the event of a shutdown, different parks may adopt different policies, according to officials, who spoke on the condition of anonymity to discuss private deliberations. Some free-standing gift shops could remain open, while those in parks' visitor centers would have to close because there would be no park staff to let workers in.
Easily accessible scenic areas would stay open, they said, while isolated backcountry trails could close because of safety risks and fewer staff members in position to respond in emergencies.
A privately run lodge could stay open, so long as it did not rely on the Park Service for snow and trash removal, the officials said. And visitors could be encouraged to bring their own grocery bags to collect trash.
Acting Glacier National Park Superintendent Eric Smith said visitors would be able to drive along the eight miles of the park that are open, and they would be able to snowshoe and cross-country ski. The visitor center, lodges and gift shops would be closed, but toilets would be accessible, and two law-enforcement rangers would be on duty to respond to emergencies.
"People who have driven a long way to see the scenery won't be completely disappointed," Smith said.
Lena Sun, Peggy McGlone, Moriah Balingit and Sari Horwitz contributed to this report.