What you need to know about the government shutdown showdown

— -- President Donald Trump's threat Tuesday to force a government shutdown over the battle to pay for his promised southern border wall raised concerns on Capitol Hill among legislators who fear that the fight for funds could turn into a hostage situation.

It's been four years since the last federal government shutdown and would be the first time under Republican control of the White House, Senate and House.

Here's what you need to know about a possible government shutdown:

What happens when the government shuts down?

A shutdown occurs when full-year or interim appropriations for federal agencies and programs expire. If additional funding isn’t approved by the president and Congress in time to continue government operations, the government shuts down. Under the Antideficiency Act, agencies are required to stop operating, with some exceptions.

During a shutdown, several hundred thousand federal employees are furloughed, government services like school lunch programs are halted, national parks are closed and several sectors of the economy, like tourism, are impacted. Depending on the duration of the shutdown, delivery of millions of Social Security benefit checks could be temporarily halted.

Why might it happen this time?

Lawmakers have until Sept. 30 -- 12 legislative days after their return from August recess -- to pass funding through both chambers of Congress. Otherwise, the government runs out of money on Oct. 1.

The president and some conservatives in the House Freedom Caucus are insistent that a spending bill contain wall funding. Without it, some conservatives may refuse to support any budget resolution. With it, Senate Democrats may filibuster.

A shutdown under Trump would be the first shutdown under unified Republican control of government.

The budget resolution could also become part of the negotiations over raising the nation’s debt ceiling, an issue coming to a head at nearly the same time. The Treasury Department has estimated that sometime in October the government will reach its borrowing limit.

Where does President Trump stand?

President Trump told the crowd at a Phoenix campaign rally on Aug. 22 that he’s willing to “close down our government” in order for his wall on the southern border to be built.

“Let me be very clear to Democrats in Congress who oppose the border wall and stand in the way of border security, you are putting all of America's safety at risk." said Trump.

A “Good Shutdown”

Trump’s message in Phoenix echoes a stance he took when he was unhappy with the last bipartisan spending deal reached in May. The president at that time tweeted: “Our country needs a good ‘shutdown’ in September to fix mess!”

Office of Management and Budget Director Mick Mulvaney, when asked to describe what the president meant by a “good shutdown," said: “It would be one that fixes this town. The one that drives the message back home to people, it was as broken as they thought that it was when they voted for Donald Trump.”

Mulvaney said that if Democrats don’t “behave better”, a shutdown may be inevitable.

What are Congressional Leaders Saying?

Speaker of the House Paul Ryan, R-Wis., said that while he agrees on the need for a border wall, he doesn’t think a shutdown is “in our interest."

“I don’t think a government shutdown is necessary and I don’t think most people want to see a government shutdown,” Ryan said in a press conference in Oregon on August 23. The Speaker has expressed his interest in passing a stopgap measure to give lawmakers more time to fund the government.

Senate Majority Leader Mitch McConnell, R-Ky. said in a statement Aug. 23 that he is working with President Trump on shared goals, including funding the government. He made no mention of the border wall.

What happened during the government shutdown in 2013?

The most recent funding gap started on Oct. 1, 2013, the first day of fiscal year 2014; a 16-day shutdown followed.

During that time, 800,000 federal workers stopped working, school lunch programs were curbed, millions of tourists saw vacations disrupted as national parks closed. Capitol Police officers worked but did not receive a paycheck. Government grants dried up as furloughs at military bases squeezed local charities. Catholic Charities in Biloxi, Miss. reported being flooded with requests for food and assistance.

During the shutdown, families of deceased service members also didn’t receive so-called “death benefits” -- a $100,000 payment usually issued within three days of notification of death. A charity called the Fisher House Foundation stepped in to provide funding for funeral arrangements and travel and the Pentagon eventually reimbursed the charity.

The Social Security Administration issued a warning that if the shutdown stretched into Day 17, checks would not be sent out. Sixty million Americans would have been impacted, each receiving an average of $280 a week in benefits.

An analysis by the Standard and Poor's ratings agency concluded the shutdown effectively took $24 billion out of the economy. It was the first shutdown to occur in over 17 years. The longest shutdown lasted 21 days during fiscal year 1996.