WASHINGTON — President Bush's warning that Social Security faces a looming financial crisis is based on the assumption that the U.S. economy will grow by 1.8 percent each year, on average, for most of the next 75 years.

Since 1950, the U.S. economy has grown, on average, by 3.5 percent a year.

If the economy continued to grow at a rate anywhere near the past pace, Social Security wouldn't face a financial crisis, although it would require small adjustments to balance income and costs. Many economists think that such a healthy rate of economic growth is more likely than not. They warn that Bush is making a case for Social Security restructuring based on improbably pessimistic forecasts about everything from immigration to fertility rates.

"The last couple of decades should have been a humbling lesson that we really cannot predict the future," said James Glassman, senior economist at JP Morgan Chase in New York. "Logic can be debated. It is not God's truth and set in stone. It is a debatable set of assumptions they are making."

By law, Social Security trustees must anticipate the retirement system's financial needs over 75 years. Because so many unpredictable variables could affect the U.S. economy over time, predicting a crisis 50 or 75 years from now is little more than a shot in the dark.

Social Security trustees eight years ago projected that dismal annual economic growth averaging 1.3 percent would create a financial crisis for the system by 2029. At that date, Social Security wouldn't be able to pay full benefits to retirees. Trustees assumed that U.S. economic growth would slow to 2 percent annually between 1998 and 2004, and remain weak afterward.

Instead, the U.S. economy grew by 3 percent annually, on average, in that period. That led trustees to push back their estimated "crisis" date by 13 years, to 2042.

Fast-forward to 2004. Because short-term growth turned out stronger than assumed, the system's trustees boosted their long-term forecast. They now assume that annual economic growth from 2015 to 2080 will average 1.8 percent. That's better than 1.3 percent, but still far worse than what most economists forecast.

The nonpartisan Congressional Budget Office, on the other hand, uses slightly more optimistic assumptions on inflation and employment, and estimates that Social Security will be able to pay full benefits to retirees until 2052.

The trustees base their conservative economic forecast on assumptions that employment, average hours worked and productivity — hourly output per worker — will show significant downturns. Many economists disagree on each point.

"I think it is more likely that these assumptions are too conservative," said Jason Furman, a professor at Harvard University and former economic adviser to President Clinton.

Furman points to fertility rates, an important indicator of the size of the future work force. They have risen in the United States since 1976 and stabilized at two children per woman. Some economists also question their assumptions about immigration, which affects work-force size. From 1992 to 2002, when the U.S. economy was growing, legal immigration averaged 840,000 people a year. Trustees assume the flow will return to something near the number that entered from 1977 to 1990: 580,000 a year. They project that legal immigrants will average 600,000 annually from 2024 through 2080.

But Glassman of JP Morgan argues that America's experience suggests a different conclusion. Rising immigration, he said, partly explains why the U.S. economy grew by 3.2 percent annually since 1992.

"What we just learned is, if we do well, we are a magnet for workers," Glassman said.

Moreover, conventional ways of estimating the economic future, especially the complicated issue of productivity, may be outdated. In the 1990s, many economists assumed that an aging U.S. population would lead to a smaller and thus less-productive work force. But productivity accelerated by 3.8 percent in 2002, and by 3.4 percent in 2003. Technology and innovation boosted productivity. That could continue.

In a nod to critics, trustees acknowledged in their 2004 report that expert opinion is divided on productivity.

Bush has warned that Social Security faces a deficit of $3.7 trillion by 2080, and $10 trillion ultimately, unless its finances are overhauled. Those numbers are based on the trustees' economic assumptions. Because the assumptions are dubious, many analysts are skeptical of Bush's warnings.

"These numbers in the trillions are completely meaningless," said Mark Weisbrot, co-author of the 2000 book "Social Security: The Phony Crisis" and co-director of the Center for Economic and Policy Research, a liberal research unit in Washington.

What Bush omits, Weisbrot said, is that the trustees also project that the U.S. economy in 2080 will generate a gross domestic product — the total value of all goods and services produced — of a whopping $567 trillion. In that context, a Social Security system deficit of $3.7 trillion would be only seven-tenths of 1 percent of GDP, and entirely manageable.