IMPEACH BUSH

So, how bad could this get?

Until a few months ago, it was accepted wisdom that the American economy functioned far more smoothly than in the past. Economic expansions lasted longer, and recessions were both shorter and milder. Inflation had been tamed. The spreading of financial risk, across institutions and around the world, had reduced the odds of a crisis.

Back in 2004, Ben Bernanke, then a Federal Reserve governor, borrowed a phrase from an academic research paper to give these happy developments a name: "the great moderation."

These days, though, the great moderation isn't looking quite so great — or so moderate.

The recent financial turmoil has many causes, but they are tied to a basic fear that some of the economic successes of the last generation may yet turn out to be a mirage. That helps explain why problems in the American subprime mortgage market could have spread so quickly through the world's financial system. On Tuesday, Mr. Bernanke, who is now the Fed chairman, presided over the steepest one-day interest rate cut in the central bank's history.

The great moderation now seems to have depended — in part — on a huge speculative bubble, first in stocks and then real estate, that hid the economy's rough edges. Everyone from first-time home buyers to Wall Street chief executives made bets they did not fully understand, and then spent money as if those bets couldn't go bad. For the past 16 years, American consumers have increased their overall spending every single quarter, which is almost twice as long as any previous streak.

Now, some worry, comes the payback. Martin Feldstein, the éminence grise of Republican economists, says he is concerned that the economy "could slip into a recession and that the recession could be a long, deep, severe one." In Monday's Democratic presidential debate, Barack Obama made the same argument: "We could be sliding into an extraordinary recession," he said.

In the next breath, of course, Mr. Obama suggested that the right policies might still help, while Mr. Feldstein has said that a recession isn't yet a sure thing. And much of the great moderation is real. Computers allow managers to run their businesses more efficiently and avoid some of the wild swings. The Fed and central banks in other countries have learned from their past mistakes.

But a recession is now more likely than not. It may well have started already. The Philadelphia Fed reported Tuesday that the economy shrank in 23 states last month, including Ohio, Missouri and Arizona, and was stagnant in seven others. California and Florida, with their plunging home values, may soon join the recession list.

The bigger question is how severe the recession will be if it does come to pass. The last two, in 1990-1 and 2001, have been rather mild, which is a crucial part of the great moderation mystique. There are three reasons, though, to think the next recession may not be.

First, Wall Street hasn't yet come clean. Even after last week, when JPMorgan Chase and Wells Fargo announced big losses in their consumer credit businesses, financial service firms have still probably gone public with less than half of their mortgage-related losses, according to Moody's Economy.com. They're not being dishonest; they just haven't untangled all of their complex investments.

"Part of the big uncertainty," Raghuram G. Rajan, former chief economist at the International Monetary Fund, said, "is where the bodies are buried."

As Mr. Rajan pointed out, this situation is more severe than the crisis involving Long Term Capital Management in the late 1990s. That was a case in which a limited set of bad investments, largely at one firm, had the potential to drive down the value of other firms' holdings in the short term. Those firms then might have stopped lending money because they no longer had the capital to do so. But their own balance sheets were largely healthy.

This time, the firms are facing real losses, which will almost certainly curtail lending, and economic growth, this year.

The second problem is that real estate and stocks remain fairly expensive. This shows just how big the bubbles were: despite the recent declines, stock prices and home values have still not returned to historical norms.

David Rosenberg, a Merrill Lynch economist, says that the stock market is overvalued by 10 percent relative to corporate earnings and interest rates. And remember that stocks usually fall more than they should during a bear market, much as they rise more than they should during a bull market.

The situation with house prices looks worse. Until 2000, the relationship between house prices and rents remained fairly steady. The same could be said about house prices relative to household incomes and mortgage rates. But the boom of the last decade changed this entirely.

For prices to return to the old norm, they would still need to fall 30 percent across much of Florida, California and the Southwest and about 20 percent in the Northeast. This could happen quickly, or prices could remain stagnant for years while incomes and rents caught up.

Cheaper stocks and houses will benefit many people — namely those who don't yet own a home and still have most of their 401(k) investing in front of them. But the price declines will also lead directly to the third big economic problem.

Consumer spending kept on rising for the last 16 years largely because families tapped into their newfound wealth, often taking out loans to supplement their income. This increase in debt — as a recent study co-written by the vice chairman of the Fed dryly put it — "is not likely to be repeated." So just as rising asset values cushioned the last two downturns, falling values could aggravate the next one.

"What people have done is make an assumption that these prices could continue rising at the rate they had been," said Ed McKelvey, an economist at Goldman Sachs. "And that does seem to have been an unreasonable assumption."

Certainly, there are some forces to push in the other direction. Outside of Wall Street, corporate balance sheets remain remarkably strong, while the recent fall in the dollar will help American companies to sell more goods overseas.

But it's hard not to believe that the economy will pay a price for the speculative binge of the last two decades, either by going through a tough recession or an extended period of disappointing growth. As is already happening, banks will become less willing to lend money, households will become less willing to spend money they don't have and investors will become more alert to risk.

Welcome to the new moderation.

E-mail: leonhardt@nytimes.com