Explainer-Why would the US government shut down?

By Andy Sullivan

WASHINGTON (Reuters) -The U.S. Congress has until the end of September to pass legislation keeping the federal government open or trigger a partial shutdown.

WHY WOULD THE GOVERNMENT SHUT DOWN?

Congress is supposed to allocate funding to 438 government agencies before October 1, the start of the fiscal year. But lawmakers rarely meet this deadline and routinely pass temporary spending bills to keep the government operating while they finish their work. If they let that funding lapse, government agencies must stop all work not deemed "essential."

Democrats and Republicans are typically at odds over spending, and it is not unusual for negotiations to run right up to the point where funding is due to expire.

Those tensions have been amplified under President Donald Trump.

Since taking office, he has dismantled several government agencies, overseen the departure of hundreds of thousands of civil servants, and refused to spend billions of dollars authorized by Congress. Trump's fellow Republicans have applauded those moves, even though they erode Congress' power over fiscal matters, while Democrats have been unable to stop him.

Republicans control both the House of Representatives and the Senate, but they will need at least seven Democratic votes in the 100-seat Senate to pass spending legislation.

This gives the minority party some leverage, and some Democrats insist that any spending bill must contain safeguards to prevent Trump from cutting programs without authorization. Republicans say Trump should be allowed to run the government as he sees fit.

WHAT IS THE IMPACT OF A GOVERNMENT SHUTDOWN?

There have been 14 shutdowns since 1981, according to the Congressional Research Service, many lasting only a day or two. The most recent one was also the longest, lasting 35 days between December 2018 and January 2019, during Trump's first term in office, due to a dispute between the president and Congress over border security.

Hundreds of thousands of federal workers would be furloughed without pay and a wide range of services could be disrupted, from financial oversight to trash pickup at national parks.

Other workers deemed essential would remain on the job, though they also would not get paid. In the past, federal workers have been paid for their time off retroactively.

Shutdowns that last only a few days have little practical impact, particularly if they occur over a weekend, but the broader economy could suffer if federal employees begin missing paychecks after two weeks.

The 2018-2019 shutdown cost the economy about $3 billion, equal to 0.02% of GDP, according to the Congressional Budget Office.

This time, a prolonged shutdown would create more turmoil as Trump's trade wars and battles with the Federal Reserve have already injected uncertainty into the global economy.

WHAT FUNCTIONS ARE CONSIDERED ESSENTIAL?

Each department and agency has a contingency plan to determine which employees must keep working without pay.

The 2018-2019 shutdown furloughed roughly 800,000 of the federal government's 2.2 million employees.

In the 2018-2019 shutdown the Trump administration kept the 63 national parks open, though public restrooms and information desks were closed and waste disposal was halted.

(Reporting by Andy Sullivan; Editing by Scott Malone and Andrea Ricci)