“The economy is still strong, job growth is still healthy, the unemployment rate is low, and the global economy is booming,” said Liz Ann Sonders, chief investment strategist at the brokerage firm Charles Schwab. “But there are cracks here and those cracks seem to be gaining more of the market’s attention than the stronger components. We’ll have to see which one takes hold.”

Early yesterday, President Bush and four of his top economic and financial officials went on the airwaves to try and dispel worries about the economy. They cited a Commerce Department report that showed that the economy rebounded in the three months that ended in June; it grew 3.4 percent, which was up from a pace of 0.6 percent in the first quarter.

“And so I want the American people to take a good look at this economy of ours,” Mr. Bush said. “The world is strong — the world economy is strong. I happen to believe one of the main reasons why is because we remain strong.”

While markets did bounce up briefly early in the day, investors found little solace in the latest economic report or the soothing words from Washington.

When trading ended, the S.& P. had fallen 1.6 percent, or 23.71 points, to 1,458.95; it is now up 2.9 percent for the year after having been up 9 percent last week. The Dow fell 1.5 percent, to 13,265.47, and is up 6.4 percent for the year, about half what it was a week ago. And the Nasdaq composite index fell 37.10 points, or 1.4 percent, to 2,562.24. With several swings, the stock market resembled a roller coaster ride — though one that probably provided little amusement for many investors. The Dow, for instance, swung 254 points from its high to the nadir at which it ended the day. With a half-hour left in trading, it appeared that stocks would close only modestly down for the day before they plunged — the Dow dropped about 135 points in 25 minutes.