That would starve the economy and create doubt in the minds of investors and ordinary Americans that the United States is good for the money.
"If the confidence in the reliability of payments were cast into doubt, the consequences for the budget, the U.S. economy, the U.S. and global financial systems could be large and lasting and very damaging," Douglas Elmendorf, the current CBO director, told Congress recently.
U.S. standing in the world would be hurt. It will be very hard to justify to anyone why the world's largest economy and richest country is willfully choosing not to pay what it owes.
Markets and the economy would tumble: Economist Mark Zandi worries that the stock market could start falling by hundreds of points a day as the calendar moves closer to Oct. 17 if Congress shows no signs it will raise the debt ceiling. And it really won't help confidence if the government shutdown is still going on.
Should it get to the point that Treasury can't pay all bills in full and on time? "We're going back into recession. It's an economic Pandora's box," Zandi told CNN.