“In the past few years, the federal government has been recording the largest budget deficits since 1945, both in dollar terms and as a share of the economy. Consequently, the amount of federal debt held by the public has surged,” the nonpartisan agency’s analysts said in a grim review of the government’s budget challenges. The CBO said growing debt will be driven chiefly by an aging population that needs higher spending on health care, and by higher interest payments on the debt. Public debt will be equal to 70 percent of gross domestic product at the end of this fiscal year — up from 40 percent at the end of 2008 [-] and will be on its way to more than 200 percent of GDP by 2037.
Of course it would happen even sooner if you follow the advice of Larry Summers and Paul Krugman.
