The threat of a government shutdown looms larger every minute. But that doesn't mean the entire government will close its doors if Congress fails to pass a budget bill in time for the new fiscal year that begins Tuesday.

Up to 800,000 federal employees could be furloughed as services deemed "non-essential," such as national parks, passport offices and most regulatory agencies -- including the Environmental Protection Agency and the Securities and Exchange Commission -- are closed.

But in short, the full impact of a shutdown is not clear, at least for now. If the federal government closes for business Tuesday, non-essential employees would be furloughed without pay. "Essential" workers-- such as military personnel, border security officials and air traffic controllers -- would be told to report to work. They wouldn't receive paychecks during the shutdown but would be paid retroactively after Congress passes a government funding bill.

Since it's up to each federal agency and department to determine which of its employees are essential and nonessential -- which the government now prefers to call "excepted" and "non-excepted" -- it's uncertain exactly how many employees, and which ones, would be furloughed. But the two most recent prolonged shutdowns in the mid-1990s give a clue.

A five-day shutdown in November 1995 caused about 800,000 federal employees out of a total of 4.4 million to be furloughed, according to a new report by the nonpartisan Congressional Research Service. The second, which lasted for 21 days from December of that year until early January 1996, resulted in about 284,000 employees being forced to stay home without pay. Another 475,000 essential employees continued to work in nonpay status.

While the government isn't obliged to give furloughed employees back pay when a shutdown ends, most received belated paychecks for their time away from work during the 1995-96 shutdowns.

Members of Congress, due to their constitutional responsibilities and a permanent appropriation for congressional pay, aren't subject to furloughs, though many of their staffers would be.

The president and all presidential appointees also would be required to work through a shutdown - and thus be paid.

The judiciary branch, including the Supreme Court, has a contingency fund that would allow it to stay open for about 10 working days into a shutdown. The Supreme Court is scheduled to open its 2013-14 session Oct. 7.

Mail would be still be delivered, Social Security checks still be would sent out and airports would remain open. New Medicare applicants likely would have to wait to be enrolled, though a shutdown isn't expected to affect medical services for those already in the program.

But a shutdown likely would shutter national parks, museums and monuments, including all Smithsonian Institution museums, as well as passport offices and visa application centers. The fate of the National Zoo's popular Panda Cam, which is recording the movements of the new baby panda, has not been decided.

Other federal operations and activities affected during the 1995-96 shutdowns, according to CRS, include:

• The National Institutes of Health stopped accepting new patients into clinical research programs. Hotline calls concerning diseases also were not answered.

• The cancellation of the recruitment and testing of federal law enforcement officials, including a delay in the hiring of 400 border patrol agents.

• Multiple services were curtailed for military veterans, ranging from health and welfare to finance and travel.

• Of $18 billion in Washington-area federal contracts, an estimated $3.7 billion -- more than 20 percent -- was hurt by the funding lapse. Contractors reportedly furloughed some employees without pay.

• The National Institute of Standards and Technology was unable to issue a new standard for lights and lamps scheduled to be effective Jan. 1, 1996, possibly resulting in delayed product delivery and lost sales.

• The closure of 368 National Park Service sites resulted in a reported loss of 7 million visitors, while national museums and monuments reported 2 million fewer visitors.

Because Congress treats the Washington, D.C., government as a federal agency during shutdowns, Mayor Vincent Gray was required to submit a plan to the federal Office of Management and Budget detailing which of the city's operations would close and stay open. In a surprising move Wednesday, he declared all of the District's 30,000-plus employees essential. The OMB hasn't commented on Gray's proposal.