NEW YORK (CNNMoney.com) -- The countdown to a possible government shutdown is on.
If lawmakers don't pass a funding extension by March 4, the government will shift to performing essential operations only. That could mean that government workers would stay home, national parks and museums would close and cleanup at toxic waste sites would stop.
It's difficult to predict how the current government would respond to a shutdown, because each federal agency is responsible for crafting and updating its own "shutdown plan."
Those plans are not made public. But the past offers some clues.
The last time the federal government went dark was for five days in November 1995 and another 21 days, ending in January 1996, during the Clinton administration.
As a result, the government closed 368 National Park Service sites, along with national museums and monuments, according to a Congressional Research Service report.
In addition, 200,000 passport applications went unprocessed, and toxic waste cleanup work at 609 sites stopped, according to the same report. The National Institutes of Health stopped accepting new clinical research patients, and services for veterans, including health care, were curtailed.
A shutdown would also result in the furlough of hundreds of thousands of federal employees, but the government would keep essential services -- like air traffic control, and the national security apparatus -- in full operating mode.
And yes, the mail will still be delivered.
Still, a large number of federal workers would be asked to stay home.
"When we go into a funding hiatus, this restricts activities ... and agencies are going to trim back the number of people who actually show up at the office," Denise Fantone, director of strategic issues at the Government Accountability Office, told CNNMoney the last time a shutdown seemed likely. "The first thing any agency is going to do is pull out their list of essential personnel."
While on furlough, federal employees won't receive a paycheck, nor would government contractors. Federal employees will eventually receive back pay, but contractors won't be so lucky.
The longer the shutdown goes on, the trickier it gets for agencies to define "essential personnel."
During the last long-term shutdown, the Social Security Administration kept enough staff in place to ensure benefits were paid out, but new claims weren't being processed. As the shutdown wore on, the agency recalled workers to start processing new claims.
What has to happen
With Congress in recess this week, lawmakers will have only four working days to pass a spending bill before the current temporary measure expires.
The good news, if you can call it that, is that Congress has come down to the wire many times before and has usually managed to pass a funding bill.
The starting point for negotiations is a bill approved by the House just before dawn on Saturday that would cut $60 billion in federal spending for the current fiscal year.
But that bill would set spending below levels acceptable to Democrats. President Obama said last week he would veto the House measure should it reach his desk.
If lawmakers can't strike a long-term deal that both Senate Democrats and conservative House Republicans will go along with, Congress might resort to passing another short-term measure that would fund the government while negotiations continue on a deal for the rest of the fiscal year.
Typically, lawmakers pass 12 appropriation bills for the president's approval. Those bills give federal agencies the legal authority to spend and conduct business.
American Airlines stock returned to Wall Street on Monday, emerging from bankruptcy after completing the merger with US Airways to form the world's largest airline company. More
The American Dream is supposed to mean that through hard work and perseverance, even the poorest people can make it to middle class or above. But it's actually harder to move up in America than it is in most other advanced nations. More
Holding on to Snowden's NSA documents "can get scary," says the New York Times' top tech executive. More
You have to search the fine print on Tegu's toy block set to find any hint of the company's plan to make one of Central America's poorest cities a better place. More
As usual, Congress has left all the year's major fiscal decisions to the last minute. More