You don't even want to consider the stress on US Economy should the US Federal and State Governments have to restrict themselves to a 'balaanced' budget. Taxes would go through the roof and that's not even considering the fact that they are not, nor can, fund the shortfall in pension/healthcare costs they are on the hook for in coming years.

If investors stop purchasing US Debt instruments the only source of revenue then becomes US Businesses and citizen taxpayers.

Mind you, while in somewhat better condition there are a lot of Canadian pension/healthcare benefits not properly funded on an actuarial basis and that won't be much fun to run into either.