There is no guarantee that the United States will keep it’s AAA rating. Even after Washington has spent the last few days and hours scrambling to put together something to ensure they do not default on payments. Investors understand that there is a fundamental problem in Washington. They spend too much.
This is why there is no promise on keeping the AAA rating. But, if the U.S. is downgraded, it certainly would not be as catastrophic as the fear-mongers would lead you to believe. In the past, other countries who have lost their AAA ratings have been able to recover much quicker.