A look at how key government sectors would be affected if the White House and congressional negotiators are unable to strike a budget deal by Friday:
--Museums and parks: The Smithsonian Institution, the National Zoo and national parks around the country would close.
--National security and law enforcement: Pay to U.S. troops would be delayed and an uncertain number of civilian Defense Department employees would be furloughed. Ongoing military operations, including those in Afghanistan, Iraq and Libya and earthquake assistance to Japan, would not be interrupted. The monitoring of terrorist watch lists, and intelligence collection and analysis would continue. The government would still monitor borders and coastlines, guard prisoners, conduct criminal investigations and provide law enforcement.
--Transportation and air safety: The nation's 15,700 air traffic controllers would keep working, as would many of the Federal Aviation Administration's 6,100 technicians who maintain the air traffic control system. Most of the National Highway Traffic Safety Administration, which issues auto recalls and makes grants to states for safety campaigns, would close.
--Taxes and government-backed loans: The Internal Revenue Service would not process paper returns; it was unclear whether taxpayer help lines would be staffed. The filing deadline remains April 18. Tax audits would be suspended. The Federal Housing Administration would stop guaranteeing loans. Loans to small businesses would be suspended.
--Social Security and Health: Social Security payments would continue to be delivered and applications for benefits would continue to be processed. Medicare would still pay medical claims for its 48 million recipients. Payments to doctors, hospitals and other service providers could be delayed, however, should a shutdown continue for several months. Medical research at the National Institutes of Health would be disrupted. Patients at NIH's hospital in Bethesda, Md., would continue to receive care, but new patients could not be admitted. No new studies of drugs or other treatments could begin.