That is a drop of nearly $150 billion, an amount much larger than the value of the tax rebates the government has sent to households this year in an effort to spur economic activity.

Financial industry executives say tighter credit from major banks represents a swing back to a realistic assessment of risk, after years of handing out money with abandon. Those practices produced a mortgage crisis whose losses could reach $1 trillion, by many estimates.

“Before, they wouldn’t verify income and they were loose on the valuations of collateral,” said John W. Kiefer, chief executive of First Capital, a private commercial lender. “Now they’re tightening down on the ability to repay. They go off the reservation, and now they come back to basics. It’s preservation for many of them at this point. It’s survival.”

But if the newfound caution of American banks is prudent in the long run, the immediate impact is amplifying the troubles with the economy. The Federal Reserve has been lowering interest rates aggressively to make money flow more loosely and to spur economic activity.

The financial system is not going along: As banks hold on to their dollars, mortgage rates are climbing. So are borrowing costs for corporations.

Some suggest that the banks, spooked by enormous losses, have replaced a disastrously indiscriminate willingness to hand out money with an equally arbitrary aversion to lend  even on industries that continue to grow.

“There’s been a lot of disruption in the credit market, and a lot of traditional lenders have really tightened up,” said Gregory Goldstein, president of Macquarie Equipment Finance, which leases computer gear and other technology to companies. “Before, some of the standards they lent on were weak, but we think they have overshot and gone too far on the other end.”