Even if the government managed to make its interest payments, fears about a default could cause investors to dump Treasurys and send U.S. borrowing rates soaring.
Here are questions and answers about the government's borrowing limit:
Q. What exactly is it?
A. The borrowing limit is a cap on how much debt the government can accumulate to pay its bills. The government borrows in most years because its spending has long exceeded its revenue. The first borrowing limit was enacted in 1917. Since 1962, Congress has raised the borrowing limit 77 times. It now stands at $16.7 trillion.
Q. When will we reach the limit?
A. The national debt actually reached the limit in May. Since then, Treasury Secretary Jacob Lew has made accounting moves to continue financing the government without further borrowing. But Lew says those measures will be exhausted by Thursday. The government will then have to pay its bills from its cash on hand — an estimated $30 billion — and tax revenue.
Q. So what happens after Thursday?
A. The government could pay its bills for a few days. But sometime between Oct. 22 and Oct. 31, the cash on hand and tax revenue wouldn't be sufficient, according to the Congressional Budget Office. The date isn't exact because it's impossible to foresee precisely how much revenue the government will receive and when.
Q. When it runs out of cash, does the government default?
A. No, not right away. A default would occur if the government fails to make a principal or interest payment on any of its Treasurys. The first interest payment after Thursday's deadline is a $6 billion payment due Oct. 31.
Many experts think that to avoid a default, Treasury would make payments on the debt its top priority.
"We believe the government would continue to pay interest and principal on its debt even in the event that the debt limit is not raised, leaving its creditworthiness intact," says Moody's Investors Service, a credit rating agency.
But that is the subject of intense dispute in Washington. The House has approved a bill to require such "prioritization." The Senate hasn't passed it, though. And President Barack Obama has threatened to veto it.