The US bond ratings are downgraded, it is the beginning of "teh suck". The US will be forced to offer higher interest rates to convince folks to buy said bonds. Higher the interest, the more of the budget goes to paying off interest. If it continues without being checked (which it probably and hopefully would), the cycle continues until it hits junk status. Then sane people stop buying them near completely.
When it hits junk status, the government either defaults on the debt or goes for hyperinflation.