(Reuters) - If President Donald Trump and the U.S. Congress fail to cut a spending deal by Friday, about 25 percent of federal operations will run out of money, causing a partial government shutdown.

FILE PHOTO: The U.S. Capitol is pictured in Washington, U.S., November 13, 2018. REUTERS/Al Drago/File Photo

Compared to some previous shutdowns, this one would be fairly minor, but Americans trying to vacation over the holidays in the national parks might be disappointed to find them closed.

Behind the showdown playing out in Washington is Trump’s demand for $5 billion to begin building a wall on the U.S.-Mexico border. Trump initially promised that Mexico would pay for the wall, but now he wants U.S. taxpayers to pay for it.

Democrats and some Republicans oppose it, arguing a costly wall would not do enough to stop illegal immigration. Here are more facts about the battle:

THE MONEY

Congress needs to approve legislation to spend about $450 billion to fund the departments of Homeland Security, Justice, Agriculture, Commerce, Interior and others, as well as provide funds to help states and localities deal with recent natural disasters.

In recent months, Congress has already approved nearly $1 trillion in spending for military, education, health, energy, veterans, labor and related programs through next Sept. 30, the end of the federal fiscal year. That represents about 75 percent of government activity.

WHAT HAPPENS IN A ‘SHUTDOWN?’

The U.S. government never totally closes, even if Washington reaches a budget impasse. Workers deemed “essential” to the federal mission must still perform their duties.

For instance, even though the Department of Homeland Security would run out of funding on Friday if Congress and Trump do not act, its border agents and those working in the interior of the country would still be on the job.

Transportation Security Administration screeners at airports and FBI agents would also keep working. And, obviously, NASA’s part of the orbiting International Space Station would not be reeled back to Earth.

Federal parks would close, unless the Trump administration finds a way to declare them an essential service.

Workers classified as not essential to public safety at the unfunded agencies would be furloughed. Both they and essential employees would not get paychecks until the dispute is resolved.

POSSIBLE OUTCOMES

- An impasse that leaves about 25 percent of the federal government unfunded for an undetermined period of time as Trump holds firm on his demand for wall funding;

- Enactment of legislation continuing funding, either for a short period or through September, for these agencies at the previous year’s levels, without money for Trump’s border wall;

- Enactment of legislation at new, fiscal year 2019 levels, with or without wall funding;

- A shutdown for around two weeks that ends at the beginning of January, when Democrats take control of the House of Representatives. They likely would pass a funding bill with no wall money, send it to the Senate and hope that Republicans in control there say yes.

A LITTLE HISTORY

Last January, Washington experienced a partial government shutdown over a weekend as politicians fought over wall funding and whether to help immigrants who were brought into the country illegally as children, known as “Dreamers.”

A second gap in funding lasted only a few hours one February night. In both cases, there was negligible impact on government services.

The United States endured a much more difficult budget impasse, which lasted more than two weeks, in October 2013. That was when a group of conservative Republicans tried to use annual funding bills to repeal the Obamacare healthcare law. Their attempt ended in failure.