The debt gap is "something that keeps me awake at night," Obama says.

He pledged to cut the budget "deficit" roughly in half by the end of his first term. But "deficit" just means the difference between government receipts and spending in a single budget year.

This year's deficit is now estimated at about $1.85 trillion.

Deficits don't reflect holdover indebtedness from previous years. Some spending items — such as emergency appropriations bills and receipts in the Social Security program — aren't included, either, although they are part of the national debt.

The national debt is a broader, and more telling, way to look at the government's balance sheets than glancing at deficits.

According to the Treasury Department, which updates the number "to the penny" every few days, the national debt was $11,518,472,742,288 on Wednesday.

The overall debt is now slightly over 80 percent of the annual output of the entire U.S. economy, as measured by the gross domestic product.

By historical standards, it's not proportionately as high as during World War II, when it briefly rose to 120 percent of GDP. But it's still a huge liability.

Also, the United States is not the only nation struggling under a huge national debt. Among major countries, Japan, Italy, India, France, Germany and Canada have comparable debts as percentages of their GDPs.

Where does the government borrow all this money from?

The debt is largely financed by the sale of Treasury bonds and bills. Even today, amid global economic turmoil, those still are seen as one of the world's safest investments.

That's one of the rare upsides of U.S. government borrowing.

Treasury securities are suitable for individual investors and popular with other countries, especially China, Japan and the Persian Gulf oil exporters, the three top foreign holders of U.S. debt.

But as the U.S. spends trillions to stabilize the recession-wracked economy, helping to force down the value of the dollar, the securities become less attractive as investments. Some major foreign lenders are already paring back on their purchases of U.S. bonds and other securities.

And if major holders of U.S. debt were to flee, it would send shock waves through the global economy — and sharply force up U.S. interest rates.

As time goes by, demographics suggest things will get worse before they get better, even after the recession ends, as more baby boomers retire and begin collecting Social Security and Medicare benefits.

While the president remains personally popular, polls show there is rising public concern over his handling of the economy and the government's mushrooming debt — and what it might mean for future generations.

If things can't be turned around, including establishing a more efficient health-care system, "We are on an utterly unsustainable fiscal course," said the White House budget director, Peter Orszag.

Some budget-restraint activists claim even the debt understates the nation's true liabilities.

The Peter G. Peterson Foundation, established by a former commerce secretary and investment banker, argues that the $11.4 trillion debt figures does not take into account roughly $45 trillion in unlisted liabilities and unfunded retirement and health-care commitments.

That would put the nation's full obligations at $56 trillion, or roughly $184,000 per American, according to this calculation.