And who would be affected?
In a government shutdown, spending for essential functions related to national security or public safety would continue along with benefit programs such as Medicare health insurance and Social Security retirement benefits for seniors.
But civilian federal employees -- from people who process forms and handle regulatory matters to workers at national parks and museums -- would be furloughed.
The last government shutdown ran from December 16, 1995, to January 6, 1996, putting about 800,000 federal workers on furlough.
Here is a roundup of the expected impact of a shutdown.
Photo: NASA Johnson Space Center control room in Houston, Texas.
Up to 1 million U.S. federal workers could face furloughs without pay beginning on Oct 1.
Most federal agency workers would be furloughed, but a small number of "excepted" employees must continue to work. These include security workers such as air traffic controllers and prison guards. Congressional staffers could work if requested by the lawmaker or committee that employs them.
Congress has previously paid federal workers for their furlough days.
Federal workers could face penalties if they tried to do any work during the furlough.
Read more. What Would Happen If The Government Shuts Down?