WASHINGTON (AP) — More than a third of federal workers would be told to stay home if the government shuts down, forcing the closure of national parks from California to Maine and all the Smithsonian museums in the nation's capital. Workers at the State Department's Bureau of Consular Affairs wouldn't be around to process visa and passport applications, complicating the travel plans of hundreds of thousands.

These would be just some of the effects of a government shutdown that furloughs 800,000 of the nation's 2.1 million federal workers. It could hit as early as Tuesday if a bitterly divided Congress fails to approve a temporary spending bill to keep the government running.

Supervisors at government agencies began meetings Thursday to decide which employees would continue to report to work and which would be considered nonessential and told to stay home under contingency plans ordered by the Office of Management and Budget.

Details about shutdown plans for each agency were expected to be posted on the OMB and individual agency websites by Friday afternoon, according to union officials briefed on the process. Formal furlough notices would be sent on Tuesday, the beginning of the new fiscal year.

"Fifty percent of our members may be locked out of work altogether during this shutdown," said J. David Cox Sr., president of the American Federation of Government Employees. "Half will be expected to continue to work without a paycheck."

Not all of government would cease to operate. Services considered critical to national security, safety and health would go on as usual, such as border patrol, law enforcement and emergency and disaster assistance. Social Security and Medicare benefits would keep coming, for example, but there likely would be delays in processing new applications.

Active-duty military personnel are exempt from furloughs, as are employees of the U.S. Postal Service, which doesn't depend on annual appropriations from Congress.

Union officials said preparations for a possible shutdown have created anxiety and uncertainty among federal workers and among those who have an expectation of government services.

"Federal agencies have had to devote time and resources to develop yet another crisis plan, distracting agencies from their critical missions," said Colleen Kelley, president of the National Treasury Employees Union. "And, if the government shuts down, the public will be further harmed by the loss of vital services people need and depend upon."

The last shutdown, which took place during the Clinton administration, lasted three weeks, from Dec. 16, 1995, to Jan. 6, 1996.

At the Smithsonian, a majority of the 6,400 employees at 19 museums would be furloughed, said spokeswoman Linda St. Thomas. Museum doors would remain closed as of 10 a.m. Tuesday, ruining vacation plans for thousands of tourists expecting to see the National Air and Space Museum or view art at one of the museum's galleries.

The National Park Service was expected to announce the specific impact of a shutdown on Friday. A contingency plan prepared in 2011 — the last time a shutdown loomed — said all 401 of the country's national parks would close and cease activities except for those necessary to respond to emergencies.

Federal courts plan to keep operations going for at least 10 business days in the event of a shutdown — roughly through Oct. 15 — using fees and other funds. But after that, only essential work would continue and each court would determine what staff is needed, according to a Sept. 24 memo from U.S. District Judge John Bates, director of the Administrative Office of the United States Courts.

Federal jury trials should continue as necessary, the memo said, and staff performing essential work at federal courts would report to work without getting paid. They would be paid when appropriations were restored.

During the 1995-96 shutdown, 20,000 to 30,000 applications by foreigners for visas went unprocessed each day, while 200,000 U.S. applications for passports went unprocessed, according to a report by the Congressional Research Service. The report also described delays in the processing of firearm applications by the Bureau of Alcohol, Tobacco, and Firearms.

The Environmental Protection Administration would essentially be closed to most of its approximately 17,000 employees, except for those involved in shutting down systems, tasked with emergency cleanups, or doing legal work in ongoing federal cases, said John O'Grady, president of the local union of EPA employees in Chicago.

NASA is still working on shutdown plans, but the agency doesn't have a launch scheduled until Nov. 6, spokesman Bob Jacobs said. Nearly all but a few hundred of the space agency's 18,000 employees would be furloughed under a contingency plan outlined in 2011.

In past shutdown threats, the space agency considered essential the operations of the International Space Station, where astronauts and cosmonauts live, and planned to continue supporting the mission if the government had shuttered, Jacobs said.