A debt default would make collapse of Lehman look like a sedate, um, Tea Party. Seriously. US bond prices would tank, markets worldwide would get bizarre, and everything that is in some way pegged to the price of bonds (and their number is legion) would go haywire. There would be massive defaults on other debt, humungous margin calls, and the stock market would consider itself lucky if it only dropped 500 points the first day.
But the carnage would continue for weeks after that. A debt default is not something to play chicken with. You are fucking with forces way beyond the ability of anyone to control.