A government shutdown would leave wide latitude to President Obama to decide exactly which federal agencies stop working and which ones are deemed "essential services" that have to keep going even through a shutdown.
But as all sides flirt with the first shutdown in nearly two decades, there are more questions than answers about what, exactly, it would look like.
The dangers range from another stall in the economy, to the potential for an international adversary to pick a fight at that moment.
"A federal government shutdown could have possible negative security implications, as some entities wishing to take actions harmful to U.S. interests may see the nation as physically and politically vulnerable," the Congressional Research Service reported last month.
Here at home, the effects of a shutdown can be judged somewhat from the last shutdowns in 1995 and 1996, when many federal employees were sent home, causing national parks to close, passport applications to bottle up and those applying for Social Security benefits to face delays with nobody to process their paperwork.
The troops, however, remained on the job, as did federal law enforcement and the court system, either being designated as essential personnel or being paid for out of fees that allowed them to keep going.
The basic rule, according to CRS, is that the government cannot incur any new discretionary spending obligations.
Two years ago, when Republicans took control of the House and they and Mr. Obama first engaged in shutdown brinkmanship, the administration had each agency work up a shutdown plan. The agencies are now in the process of updating those plans.
Much of what happens, though, is up to the president, and this administration has been reluctant to tip its hand on spending decisions over the last few years.
The Congressional Budget Office has a hard time even predicting the exact outlines of what a shutdown would look like.
"All of the government shuts down except the parts that are deemed essential services, and I think there's a great deal of latitude in that determination for the president," CBO Director Douglas Elmendorf told reporters last week. "So how much of the government stops working and how much keeps working is not something we have a good basis for predicting."
Still, he said a broader, longer shutdown will hurt the economy more than a slight, short halt to operations.
Once a shutdown is resolved, it's up to Congress whether federal workers — both those who were required to stay on the job and those who were sent home — get paid for that time.
In the 1990s shutdowns, Congress and then-President Clinton did agree to pay those workers, but that is not a requirement.
One thing that doesn't actually shut down is the new health care law. While parts of it are dependent on government employees showing up and doing their jobs, other parts are independent — such as the tax subsidies some Americans would be entitled to under the law.
"Like many other tax credits, the monies used to fund such provisions are permanently appropriated outside of the annual appropriations process. Therefore, the funds for such credits would continue to be available via this permanent appropriation during a government shutdown caused by a lapse in annual appropriations," the Congressional Research Service told Sen. Tom Coburn, Oklahoma Republican, in a memo earlier this summer.
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