Factbox: What happens in a U.S. government shutdown?

WASHINGTON (Reuters) - Democrats and Republicans in the U.S. Congress are waging a budget battle that threatens a government shutdown on October 1 unless lawmakers reach a compromise on a federal spending measure. The last shutdowns occurred during the budget battle between the Republican-controlled Congress and President Bill Clinton in late 1995. Much of the federal government was closed for five days in November 1995 and then from mid-December 1995 to early January 1996. Many U.S. agencies also made preparations ahead of a threatened shutdown in 2011, and on Tuesday the Office of Management and Budget said agencies should update their contingency plans. Here is what the public might expect if the federal government is forced into a partial shutdown: FEDERAL WORKERS Employees in all three branches of government are vulnerable to furlough, or temporary unpaid leave, although each agency makes its own final decisions, according to a report last month by the Congressional Research Service. However, some high-level employees, such as the president and presidential appointees, are not subject to furlough. Other so-called "essential" workers must work during a shutdown, some with and others without pay. For example, exempt workers include those whose work is critical for national security or public health and safety, such as air traffic controllers and border security agents, according to federal guidance. Affected workers may be able to receive pay retroactively, as has happened after past shutdowns, but such payments are not guaranteed, CRS staff said. About 800,000 federal workers were furloughed in November 1995; the next month, nearly 300,000 were furloughed and another nearly 500,000 worked without pay. Federal contractors are also likely to be affected. In the last shutdown, more than 20 percent of federal contracts, or nearly $4 billion, were involved, researchers said. Members of Congress cannot be furloughed, according to CRS. PUBLIC SAFETY For the most part, services to protect the public should be the least affected by furlough, although workers could still face "no pay" status. Guidelines direct federal agencies to continue police work and criminal investigations and operate prisons, according to CRS. Work to care for critical patients, oversee food and drug safety, and protect federal research laboratories is also considered essential. FEDERAL BENEFITS The impact on the Social Security and Medicare entitlement programs is unclear. Delays could be seen in processing new claims, congressional researchers said. In the last shutdown, the Social Security Administration initially kept fewer than 5,000 workers on the job out of more than 66,000 workers. The agency then had to call back nearly 50,000 workers to provide critical services, CRS said. Military veterans could also see delays in processing of benefits as well as service cuts affecting health, income, travel and more, according to the research service. CRIME AND COURTS The U.S. judicial branch is likely to continue to operate for a period of time using various fees it receives. Judges, key court staff and probation officers would not be furloughed, although each court would determine its own staffing needs. Supreme Court justices and other critical federal judges would be paid while other essential staff would not, CRS said. Most federal courts "generally operated with limited disruption" during the 1995-1996 shutdown, it added. Still, thousands of bankruptcy and delinquent child support cases were delayed. Federal work related to the nation's banking system and the U.S. Treasury, such as borrowing and tax collection, are also deemed critical and likely to be exempted, according to federal guidelines. TOURISM AND TRAVEL Tens of thousands of Americans' passport applications could go unprocessed along with visa applications from foreigners seeking to visit the United States, which will affect airlines and other related industries. Hundreds of National Park Service sites could close temporarily along with national monuments and museums, losing millions of visitors. Such closures can also impact neighboring communities and businesses that depend on tourism. HEALTH In the last shutdown, the National Institutes of Health stopped enrolling new patients into clinical trials and shut down disease hotlines, CRS said. The Centers for Disease Control and Prevention, which monitors foodborne illnesses and other outbreaks, also halted surveillance, it added. GOVERNMENT DATA Some U.S. economic data would likely be delayed. During the 2011 budget battles, a U.S. official said no data from the Commerce Department or from the Bureau of Labor Statistics, which handles the closely watched monthly employment report, would be released in the event of a shutdown. (Reporting by Susan Heavey; Editing by Karey Van Hall and Jackie Frank)