The U.S. is the only big country with a debt ceiling. The Post's Karen Tumulty explains why. (The Washington Post)

Visa and passport applications from almost half a million prospective travelers would sit untouched every week, and late tax filers facing an Oct. 15 deadline would find the phone lines at the Internal Revenue Service dead. The National Zoo and museums on the Mall would close to the public. The WIC program, which provides food to 8.9 million low-income women and children, would be out of money, its supporters say.

And in the District, garbage could go uncollected.

This is, in part, what a government shutdown would look like. If Congress reaches Tuesday with no approved spending plan to keep the government open, many of the day-to-day services the public sometimes takes for granted would come to a standstill, as federal agencies send home more than 800,000 employees whose jobs are not considered essential.

Managers are kicking their preparations into high gear as Congress remains deadlocked, with House Republican leaders signaling that they will reject a short-term spending plan that the Senate seems likely to pass.

Agencies are declining to talk about contingency plans, which under law must lay out scenarios for an initial period of one to five days and a second stretch if a shutdown were to continue. But officials say privately that they are dusting off blueprints they drew up during a fiscal battle in 2011 that brought the government to the brink of closing.

At the Smithsonian Institution, staff members were answering questions this week from a lot of school groups that would be shut out of Washington’s free museums. Spokeswoman Linda St. Thomas said she plans to put up a banner on the Smithsonian’s Web site if a closure becomes definite, as well as tweet the news to more than 1 million followers — an option she didn’t have during the most recent shutdowns, in 1995 and 1996.

“The Smithsonian is a big part of the visit to Washington,” St. Thomas said, “even if it’s not peak tourist season.”

EF Explore America, which organizes tours for school groups, was reaching out to 100 middle schools nationwide that have scheduled eighth-grade field trips to Washington starting Monday.

Robert Hart, the company’s vice president of operations, has been thinking up contingency plans all week. On Wednesday morning, he called Arlington National Cemetery and heard good news: The cemetery would remain open for visitors, even if the Capitol wouldn’t.

“A lot of the sites that would close would be free,” he said. “But there’s also the Newseum, Mount Vernon, the Spy Museum. . . those are marquee sites.”

If a shutdown stretches to Columbus Day weekend, he will really worry, because hundreds more tour groups are scheduled to travel to Washington then.

Closer to home, Prince George’s County public schools are notifying principals at 25 schools with field trips scheduled to the zoo, the National Air and Space Museum, the Capitol and other attractions to make other plans. Maybe.

Here's what some agencies have said about their specific plans in case of a government shutdown.

In a shutdown, the government does not stop functioning completely. Activities considered essential to national security, public safety, health and welfare continue, and employees with jobs in those areas would report to work.

So, officers at the CIA, for example, would continue to collect intelligence. Federal prisons would be staffed, and other employees on the government’s front lines, such as meat and poultry inspectors, air traffic controllers and airport baggage screeners, would stay on.

The animals at the zoo would be fed by a skeleton crew of caretakers.

Social Security, Medicare and Medicaid, and unemployment insurance — benefits considered mandatory spending — would be paid. But new applications might not be processed until the government reopened.

That’s what happened in 1996, when thousands of retirees applying for Medicare were turned away. The staff that handled new Social Security enrollments and other services was furloughed at first, then called back to work when the White House realized that processing new claims was essential.

But with no one to staff many federal offices, the lights would go off — for example, at the State Department’s Bureau of Consular Affairs, which processes 252,000 passports and 172,000 visas per week on average.

“People traveling in October have heard the news there’s a [potential] government shutdown, and we’re getting a substantial number of calls,” said Walter Rasor, floor manager at Florida-based RushMyPassport.­com, a private firm that helps expedite 1,000 passports and 500 visas a week.

Rasor said he’s telling callers not to panic: They can get a passport this week, if they pay $299 to the company and an additional $170 to the State Department for expedited processing. “I tell them we could still get it in their hands by Monday,” he said.

The Bureau of Alcohol, Tobacco, Firearms and Explosives would not be able to issue permits to gun and explosives dealers.

The government could stop insuring mortgages for first-time home buyers because the employees processing them at the Federal Housing Administration would be off the job.

“A lender would be reluctant to close a loan if it could not be insured in a timely manner,” said Brian Chappelle, a former FHA official who is now a real estate consultant. “It could really slow down closings. Lenders are pretty skittish today, and some of them are going to be conservative.”

And small businesses could find their loan applications with the Small Business Administration in limbo.

“It would be devastating for them,” said John Arensmeyer, founder and chief executive of the Small Business Majority, an advocacy group. “They’re just starting to push out of the recession.”

The country’s 401 national parks would close to the public, leaving only firefighters and law enforcement officers at work. The Environmental Protection Agency “effectively shuts down, with only a core group that are there in the event of an emergency,” Administrator Gina McCarthy told reporters at a breakfast this week.

The prospect is enraging many advocates for federal services.

“This is no way to manage the nation’s business or to care for the least among us,” said Douglas Greenaway, president of the National WIC Association, which advocates for the Special Supplemental Nutrition Program for Women, Infants and Children.

WIC centers get lines of credit every three months that they can use for food; the next is due Tuesday. Maryland’s program has been contacted by its bank with concerns about financing starting next week, he said.

The National Parks Conservation Association has been getting calls all week from families that have booked vacations at national parks next week.

“You can imagine the number of people across the country who are disappointed,” said Theresa Pierno, the group’s acting director.

Much would depend on the length of a shutdown. Some government functions could continue for a few days but then be forced to close down. Head Start, the federally funded early-childhood program, could run out of money after a few weeks because it depends on grants from the Department of Health and Human Services that must be renewed by Congress.

A spokesman for the federal courts said the system could keep operating for a short time using revenue from court fees and savings from previous budgets. Judges, as constitutional officers, must be paid no matter what. But when the fees and savings run out, it is unclear whether judges would preside over trials.

“If it’s a criminal case, will a marshal be available to bring the prisoners in?” asked David Sellers, spokesman for the administrative office of the federal courts. “Will there be a prosecutor available? The courthouses are run by the General Services Administration. Will the courthouses be open? So much is unclear.”

During the shutdowns of 1995 and 1996, Justice Department staff members’ work on 3,500 bankruptcy cases was suspended.

Government services probably would suffer even more today. Congress had approved regular appropriations for several large agencies in the 1990s, keeping their employees working. But this year, Congress has not passed a single spending bill.

The government also has new missions. In a post-9/11 era, the Bureau of Consular Affairs has a larger role in vetting visa applications from foreigners.

“Determining which foreign nationals should come to the U.S and which shouldn’t has become an entirely different procedure,” said Scott Lilly, a congressional budget expert and senior fellow at the Center for American Progress, a liberal think tank.

“It’s part of a global network of intelligence agencies,” he said. “You could have a period of weeks or even months where your data feed is not caught up with where it needs to be.”

There is also a health-care law that was not on the books in 1995: the Affordable Care Act, the center of all the controversy in Congress over funding the government. Starting Tuesday, millions of Americans are scheduled to be able to sign up for new insurance plans.

White House officials said Wednesday that a shutdown would have no effect on the rollout, because much of it is either paid for or comes from a mandatory spending fund.