The White House and Congress have until midnight on Friday to agree to a budget bill or there will be a partial shutdown of the U.S. government. The dispute is over funding for the remainder of the current fiscal year, which ends September 30.
Republicans and Democrats say they want to avoid a shutdown, which could idle hundreds of thousands of federal workers, close national parks and Washington's Smithsonian Institution museums.
If there is no deal, many official websites would darken and furloughed government workers would be required to power down their BlackBerries at 12:01 a.m. EDT (0401 GMT) on Saturday.
The federal work force numbers about 4.4 million, including members of the U.S. military.
Here are some facts about what could happen.
WHO WOULD NOT BE AFFECTED?
The military, law enforcement, FBI, prison guards, Customs and Border Protection, U.S. marshals, the U.S. Postal Service and any services deemed essential for the safety of lives and the protection of property. That would include air traffic controllers and federal disaster operations, as well as national weather and earthquake-monitoring operations.
The distribution of food stamps and other child nutrition benefits, as well as NASA satellite missions. Also, any federal service with an alternative source of funding to annual congressional appropriations, like fees or operations financed through multiyear appropriations.
HOW MANY FEDERAL WORKERS WOULD BE IDLED?
Based on the last federal government shutdown, from December 16, 1995, until January 6, 1996, about 800,000 federal workers would be furloughed, including a significant number of civilian contractors working for the Defense Department, the White House said.
The White House Office of Management and Budget said it was up to Congress to decide on back pay for furloughed workers, but the Obama administration would support reimbursement. After the 1995 and 1996 shutdowns, Congress approved back pay for furloughed employees.
WHAT WOULD HAPPEN TO THE MILITARY?
Operations in Afghanistan, Libya, Iraq and Japan would not be affected. Military personnel would not receive paychecks -- although they would continue to earn salaries -- because the government would have no money to pay out during a shutdown.
SOCIAL SECURITY AND MEDICARE
Social Security and Medicare benefits for the elderly would continue to be paid. But help lines would not be staffed.
The shutdown would occur during tax preparation and filing season -- federal income taxes are due by April 18. That would delay tax refunds to Americans who filed a paper -- rather than electronic -- tax return, or about 30 percent of the total number of returns. Electronic tax collection and refunds would continue. IRS tax audits would be suspended.
PARKS AND MUSEUMS
National parks, national forests and the Smithsonian Institution in Washington would close. But tourists in Washington, D.C., will at least be able to enjoy the city's annual Cherry Blossom Parade -- organizers said city police would organize an alternate parade route that would avoid federal land.
FEDERAL HOUSING ADMINISTRATION
The Federal Housing Administration would not be able to endorse any single-family mortgage loans or have staff available to process and approve new multifamily loans.
The White House says that as FHA single-family lending represents more than 20 percent of the overall loan volume of home purchases and refinancings, this would be a hard hit on the housing market.
OTHER SERVICES AFFECTED?
During the last shutdown, an estimated 200,000 applications for U.S. passports went unprocessed and work on more than 3,500 bankruptcy cases were suspended.
* New patients would not be accepted into clinical research at the National Institutes of Health, although clinical trials already in progress would continue.
* The Small Business Administration's approval of applications of business loan guarantees and direct loans to small businesses would stop.
* Daily life in the U.S. capital would also be affected. While schools would stay open and police and other safety services would continue, trash collection would be suspended for a week and street sweeping would cease. Public libraries and the Department of Motor Vehicles would close.
(Sources: White House Office of Management and Budget, the Congressional Research Service, congressional aides, senior Obama administration officials)