YESTERYEAR'S clichés can be correct. At least for an unpopular president, it really is the economy, stupid. Approval ratings under 40 percent usually reflect a lack of confidence that the administration knows how to manage economic problems.

That is not to say that economic issues are the only ones that matter. Iraq and Hurricane Katrina hurt.

But a look at presidential approval ratings over the last three decades shows that very low ratings tend to come when there are big doubts about domestic economic policy.

Jimmy Carter's approval rating, as measured by polls by The New York Times, first dropped below 40 percent in February 1979, as he grappled with an Iranian crisis that sent oil prices up. His call on Congress to give him the power to ration gasoline did little to assure voters that he had a way out of the problem.

Ronald Reagan's rating in the Times poll never fell below 40 percent. When it hit bottom, at 41 percent in January 1983, the country was already emerging from a recession, and the stock market was up sharply. That poll showed a strong consensus that the economy would eventually improve.

The economy was also improving in April 1992, the first time George H. W. Bush had an approval rating below 40 percent. But voters doubted his commitment to the economy, and his term ended before employment levels reflected the recovery.

For Bill Clinton, the drop below 40 percent came in the summer of 1993, as doubts about his health care plan grew and it foundered in a Congress that his own party controlled.

Mr. Clinton managed to turn his rating around, albeit not quickly. It was below 40 percent as late as December 1994, after the Democrats suffered sweeping defeats in mid-term elections. But he then changed the subject, forgetting about health care and moving to deficit reduction as his No. 1 priority.