The daunting tower of national, state and local debt in the United States will reach a level this year unmatched just after World War II and already exceeds the size of the entire economy, according to government estimates.

But any similarity between 1946 and now ends there. The U.S. debt levels tumbled in the years after World War II, but today they are still climbing and even deep cuts in spending won't completely change that for several years.

As President Obama and Republicans squabble over whose programs to cut and which taxes to raise, slow growth and a rising tide of interest payments - largely beyond their control - are making the job of fixing the budget much harder than in the past. Statehouses and governors face similar challenges.

After World War II, the federal debt - including debt purchased by the Social Security Trust Fund - hit nearly 122 percent of gross domestic product. State and municipal debt back then was minimal. By the time Dwight Eisenhower was elected president six years later, the federal government's debt had dipped to about three-fourths of GDP.

The key factor in the rapid drop in government debt, said Harvard University economist Kenneth Rogoff, was fast economic growth. Spurred by a young labor force, world-leading manufacturers, high personal savings rates, a pent-up demand for consumer goods after years of war and the Depression, and a bout of inflation, the economy grew 57 percent in six years. Thanks to sharp postwar cuts in defense outlays, federal government spending also tumbled for a couple of years.

But today the U.S. economy is in a polar opposite condition. The labor force is aging, U.S. manufacturing often lags behind Asian and European rivals, households are in hock up to their eyeballs, and consumer appetite for goods is tepid. In addition, inflation is tame and government spending locked into entitlement programs and debt service that will be hard or impossible to alter.

"We're not growing like we were after World War II, so the amount of debt you can bear and the trajectory are much worse," Rogoff said.

Moreover, today state and municipal governments are also facing fiscal woes - another difference between now and the postwar era. State and municipal governments from Sacramento to Madison to Harrisburg have racked up about $2.4 trillion in debt, or more than 15 percent of GDP.

Even if analysts leave aside the debt held by the Social Security Trust Fund, the total indebtedness of federal, state and local governments is running around 85 percent, vs. 108.7 percent in 1946.

"It's still very, very, very high," Rogoff said, "and there are a lot of things on the other side of the equation that are much worse." Moreover the debt held by Social Security, which is in surplus now, will have to be paid later as the ranks of senior citizens grow.

Robert D. Reischauer, president of the Urban Institute and former director of the nonpartisan Congressional Budget Office, said that the debt accumulated by 1946 "was for a very different purpose, which was to preserve freedom and democracy versus totalitarianism rather than to throw a huge party and put it on the credit card."

He said that state governments have also squandered much of their spending and failed to meet all their pension obligations.