If a government shutdown occurs, what actually happens?

House and Senate leaders are more than $60 billion apart on how much to spend or borrow to pay for government after March 4, when the funding for the current fiscal year runs out. If no one blinks, Washington could be headed toward a shutdown – the 16th since Jimmy Carter was president.

Most shutdowns lasted fewer than three days. One of the most famous, the standoff between President Bill Clinton and House Speaker Newt Gingrich over balancing the federal budget – lasted 21 days, from Dec. 16, 1995, to Jan. 6, 1996. That shutdown furloughed some 800,000 federal workers; delayed processing of visas, passports, and other government applications; suspended cleanup at 600 toxic waste sites; and closed national museums and monuments as well as 368 national park sites – a loss to some 9 million visitors and the airline and tourist industries that service them.

It was, as Republicans had predicted, a “train wreck,” but it hit them hardest. Americans blamed the Republican House more than Mr. Clinton for provoking the shutdown, by a margin greater than 2 to 1.

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Here’s what to expect, if Republicans and Democrats don't reconcile their differences on spending for the last half of this fiscal year:

Why must the government shut down? According to the Antideficiency Act of 1870, federal agencies and programs must cease operations if Congress and the president fail to enact funding, except in cases of emergency. The US government shut down six times between fiscal year 1977 and FY 1980, over periods ranging from eight to 17 days, according to the Congressional Research Service. From FY 1981 to FY 1995, there were nine shutdowns lasting as long as three days. Funding for the fiscal year that began Oct. 1 extends only through March 4.

Is government prepared for a shutdown? Since 1980, the White House Office of Management and Budget (OMB) has required government agencies to submit plans for an “orderly shutdown.” The plans require agency heads to “limit their operations to minimum essential activities” and to reallocate funds to avoid interruption of services as long as possible. “Those plans are obviously updated accordingly, but they’ve been around for a long time,” said White House press secretary Jay Carney at a briefing on Tuesday.

Are members of Congress exempt from a shutdown? Yes, as is the president. That's because their compensation is financed by a resource other than annual appropriations, in this case, the US Constitution. Other exempted employees are those deemed to perform emergency work involving saving lives or protecting property, including military service, law enforcement, or direct provision of medical care, according to the most recent OMB directives, released in 2010. Sens. Barbara Boxer (D) of California and Robert Casey (D) of Pennsylvania last week proposed legislation to prohibit members of Congress and the president from being paid during a government shutdown, or retroactively. According to current law, furloughed federal workers are paid retroactively. The same protection does not apply to workers under federal contract or those whose jobs are disrupted by the shutdown.

Will I continue to get my Social Security check? The Social Security Administration kept nearly 5,000 employees on the job, about 7 percent of its workforce, during the fiscal year 1996 shutdown, on grounds that its funding is determined by an entitlement formula, not annual appropriations. But SSA later recalled some 50,000 employees to handle new claims and delays.

OMB officials say they are not responding to such hypothetical questions, because they don't expect a shutdown.

“As the part of the executive branch charged with overseeing the management of the federal government, OMB is prepared for any contingency as a matter of course – and so are all the agencies," said Kenneth Baer, OMB communications director, in a statement. "In fact, since 1980, all agencies have had to have a plan in case of a government shutdown, and they routinely update them. All of this is besides the point since, as the congressional leadership has said on a number of occasions and as the President has made clear, no one anticipates or wants a government shutdown."

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