President Donald Trump, the Republican-controlled Senate and House and the congressional Democratic minority are fighting over how to extend spending for federal operations after the current authority expired on January 19.

This has plunged the US into a "Government Shutdown", the first since 2013.

Here’s what happens when government funding expires.

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What is a shutdown?

Federal government shutdowns occur when spending bills expire, and Congress and the president find themselves at an impasse.

Shutdowns have a constitutional basis, from Article I, Section 9, which says, "No Money shall be drawn from the Treasury, but in Consequence of Appropriations made by Law."

This has been reinforced by the Antideficiency Act, which stems from an 1870 law that has been revised periodically over the years. There is some leeway to continue certain federal activities that are deemed essential.

How common are shutdowns?

There have been 18 shutdowns since 1976, according to the Congressional Research Service.

However, the impact of these shutdowns has changed significantly since 1980.

Prior to 1980, many federal agencies continued to operate during funding gaps, assuming that money would be restored soon and that Congress didn’t intend for them to close down.

But in 1980 and 1981, then-Attorney General Benjamin Civiletti issued two opinions that required agency heads to suspend operations until funding began to flow again.

After 17 years without a shutdown, President Barack Obama, a Democratic-controlled Senate and a Republican-controlled House failed to reach an agreement on spending, resulting in a two-week shutdown in 2013.

How much does a shutdown cost?

"A shutdown is a huge waste of taxpayer dollars, disruptive of government operations, and simply bad management," said John Palguta, a former federal human-resources official who is now an adjunct professor at the Georgetown University Public Policy Institute.

The most thorough report examining the 2013 shutdown comes from an Office of Management and Budget analysis.

• $2.5 billion in compensation costs for furloughed workers (whose lack of pay for two weeks hampered consumer spending);

• 120,000 fewer private-sector jobs created in the first half of October;

• $500 million lost in visitor spending because of closed National Parks ;

• $11 million in lost National Parks and Smithsonian Institution revenue;

• Interest accrued on billions of dollars of payments owed to third parties that the government was unable to pay during the shutdown;

• Resources spent on putting activities in standby or maintaining them in an idle mode;

• 1.2 million Internal Revenue Service identity verification requests that couldn’t be processed, causing a delay in private-sector lending and other activities.

How does a shutdown affect ordinary Americans?

During a shutdown, some government functions may continue, either because Congress has already passed a spending bill for a particular department, because an agency is reliant on funding other than congressional appropriations, or because it is considered vital for the safety of human life or the protection of properties.

However, many activities cease for as long as a shutdown is under way. Here are just a few of the activities that have stopped during previous shutdowns, according to the Congressional Research Service:

• New patients were not accepted into clinical research at the National Institutes of Health, and hotline calls about diseases went unanswered.

• The Centers for Disease Control and Prevention ceased disease surveillance.

• Recruitment and testing of federal law-enforcement officials stopped, in one case including the hiring of 400 border patrol agents.

• National Park Service sites and national museums and monuments closed to visitors.

• Work on visa and passport services stopped.

• Inspections for food, consumer products, and workplace safety could not be carried out.

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