Actually I don't think this is the case. The US uses treasury bonds like you or I would use a credit card, they sell bonds take the money and pay bills. They pay them off by exchanging them for cash at a later date. So essentially when we're at the debt ceiling we nominally can't sell any more t-bills so the only money the government has available is money from revenues coming in. Pretty much instantly forced to live without your credit card.
Now since the current budget actually spends more than is expected in receipts, that means things that were already approved and budgeted would not have the funding to pay for them. But in the unlikely event that the debt ceiling had not been raised the government would switch to a priority system of paying (effectively cutting spending).
Its my understanding that at no time has the US ever been at risk of 'default' in the sense that a bank wants its money and the holder of the note can't pay. But it might have been in danger of suddenly withdrawing from Iraq and Afghanistan. Since at 2 B$/day (one estimate I've heard) that is over 700B$/yr of money we would probably prioitize not to spend.
Now since the current budget actually spends more than is expected in receipts, that means things that were already approved and budgeted would not have the funding to pay for them. But in the unlikely event that the debt ceiling had not been raised the government would switch to a priority system of paying (effectively cutting spending).
Its my understanding that at no time has the US ever been at risk of 'default' in the sense that a bank wants its money and the holder of the note can't pay. But it might have been in danger of suddenly withdrawing from Iraq and Afghanistan. Since at 2 B$/day (one estimate I've heard) that is over 700B$/yr of money we would probably prioitize not to spend.