The shutdown is only the fourth government closure in a quarter-century. It will only partially curb government operations.
Uniformed service members, health inspectors, and law enforcement officers are set to work without pay.
WHAT THE SHUTDOWN MEANS
MILITARY
The Defense Department said a shutdown would not affect the US military's war in Afghanistan or its operations against Islamist militants in Iraq and Syria. All 1.3 million military personnel on active duty would remain on normal duty status.
Civilian personnel in non-essential operations would be furloughed.
JUSTICE
The Justice Department has many essential workers. Under its shutdown contingency plan, about 95,000 of the department's almost 115,000 staff would keep working.
FINANCIAL OVERSIGHT
The stock market-policing Securities and Exchange Commission funds itself by collecting fees from the financial industry, but its budget is set by Congress. It has said in the past it would be able to continue operations temporarily in a shutdown.
WHITE HOUSE
More than 1000 of the 1715 staff at the White House would be furloughed, the Trump administration said on Friday. The president would be provided with enough support to carry out his constitutional duties, including staff needed for a planned trip to the World Economic Forum in Davos, Switzerland, administration officials said.
NATIONAL PARKS
The Trump administration plans to keep national parks open with rangers and security guards on duty.
TAXES
The Internal Revenue Service furloughed 90 per cent of its staff in 2013, the liberal Center for American Progress said. About $4 billion in tax refunds were delayed as a result, according to the Office of Management and Budget (OMB).
MAIL DELIVERY
Deliveries would continue as usual because the US Postal Service receives no tax dollars for day-to-day operations.
TRAVEL
Air and rail travellers did not feel a big impact in 2013 because security officers and air traffic controllers remained at work. Passport processing continued with some delays.
COURTS
The Administrative Office of the US Courts has said federal courts, including the Supreme Court, could continue to operate normally for about three weeks without additional funding.
HEALTHCARE
In 2013, the Medicare health insurance program for the elderly continued largely without disruption.
SOCIAL SECURITY
Social Security and disability cheques were issued in 2013 with no change in payment dates and field offices remained open but offered limited services. There were delays in the review process for new applicants.
LOANS
Processing of mortgages and other loans was delayed when lenders could not access government services such as income and Social Security number verification.
VETERANS
Most employees at the Department of Veterans Affairs would not be subject to furlough. VA hospitals would remain open and veterans' benefits would continue, but education assistance and case appeals would be delayed, the department said.
ENERGY
The Department of Energy said on Friday that since most of its appropriations are for multiple years, employees should report to work as normal during a shutdown until told otherwise.
A history of shutdowns
Government shutdowns are unusual but not unheard of.
The government has partially shut down three times in the past quarter-century — and far more often in decades past.
Shutdowns have led to furloughs of several hundred thousand federal employees, required many government activities to be stopped or curtailed and affected wide swaths of the economy.
During Jimmy Carter's administration, shutdowns happened nearly every year, averaging 11 days each. During Ronald Reagan's two terms in the 1980s, there were six shutdowns, typically just one or two days apiece.
Legal opinions issued in 1980 and 1981 made shutdowns more impactful. Opinions by then-Attorney General Benjamin Civiletti determined that failure to pass new spending bills required government functioning to shut down in whole or in part. Earlier "shutdowns" did not always entail an actual stop to government functioning and often were simply funding gaps will little real-world effect.
Here's a look at recent shutdowns, their causes and impact:
• October 2013: Sixteen-day partial shutdown, which came as tea party conservatives, cheered on by outside groups, demanded that language to block implementation of President Barack Obama's health care law be added to a must-do funding bill. Then-Speaker John Boehner, R-Ohio, tried to avoid a shutdown by funding the government piecemeal, but the effort faltered.The shutdown affected most government operations and resulted in the furlough of 850,000 employees, costing the government 6.6 million days of work and more than US$2.5 billion in lost productivity, according to a report by the Congressional Research Service. Boehner survived the shutdown but stepped down two years later amid conflict with the hard-right House Freedom Caucus.
• December 1995-January 1996: Republicans led by then-Speaker Newt Gingrich, intent on slashing the budget, forced a three-week shutdown in a bid to coerce President Bill Clinton to sign onto a balanced budget agreement. Republicans were saddled with the blame, but most Americans suffered relatively minor inconveniences such as closed parks and delays in processing passport applications. The fight bolstered Clinton's popularity and he sailed to re-election that November.
• November 1995: Five-day shutdown after Clinton vetoed an interim spending bill to block Medicare premium increases. Led to longer shutdown a month later.
