Congressional Budget Director Doug Elmendorf writes on the Director's Blog at the CBO web site:
Federal Debt and the Risk of a Financial Crisis
In fiscal crises in a number of countries around the world, investors have lost confidence in governments’ abilities to manage their budgets, and those governments have lost their ability to borrow at affordable rates. With U.S. government debt already at a level that is high by historical standards, and the prospect that, under current policies, federal debt would continue to grow, it is possible that interest rates might rise gradually as investors’ confidence in the U.S. government’s finances declined, giving legislators sufficient time to make policy choices that could avert a crisis. It is also possible, however, that investors would lose confidence abruptly and interest rates on government debt would rise sharply, as evidenced by the experiences of other countries.
Unfortunately, there is no way to predict with any confidence whether and when such a crisis might occur in the United States. In a brief ("Federal Debt and the Risk of a Fiscal Crisis") released today, CBO notes that there is no identifiable “tipping point” of debt relative to the nation’s output (gross domestic product, or GDP) that would indicate that such a crisis is likely or imminent. However, in the United States, the ratio of federal debt to GDP is climbing into unfamiliar territory—and all else being equal, the higher the debt, the greater the risk of such a crisis.
Read more: http://cboblog.cbo.gov/?p=1249