UPDATE: The federal government has, in fact, shut down.
On 12:01 a.m. Tuesday the federal government began to shut down because both houses of Congress have not successfully passed a spending bill -- or stopgap continuing resolution, or "CR" -- that will continue funding it for any part of the next fiscal year, which begins on Oct. 1.
Why? Most recently, the Republican-controlled House did pass a spending bill -- but included language that would delay the implementation of the Affordable Care Act for a year as well as repeal a tax on medical devices. The Democrat-controlled Senate has said it will not pass the bill; regardless, President Obama will not sign it.
A section of Congressional Republicans has persuaded their party that this battle is worth the risk. The last time House Republicans forced a government shutdown, for 21 days in 1995-96, public opinion turned venomous.
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The Postal Service will keep running, as will the military, the Food Safety and Inspection Service and the FDA; and Social Security, Medicare and Medicaid. Taxes will still be collected.
The government would still pay out unemployment benefits and the federal school lunch program would be subsidized, from excess funds from the previous fiscal year, through October, according to USA Today.
SNAP will still operate, though there will be no money to pay the administrative costs of WIC. State funds may supplement this, however.
The National Park Service, however, will close immediately and for the duration of the shutdown; as will all public museums, such as the Smithsonian. The State Department said it has some money outside of what is appropriated from Congress, so its consulates and passport-issuing functions will keep operating ... up to a point.
By late October, the Department of Veteran Affairs said it will run out of money for the pension and compensation checks it issues to more than 3.6 million veterans. The House has passed a separate bill that will pay for active-duty and reserve military.
NASA, the EPA and the Departments of Labor, Commerce and the Interior will have the highest percentage of their employees furloughed, according to The New York Times.
Every member of Congress, and the president, will still be paid.