With no sign of a compromise in Washington, investors are becoming more fearful that the U.S. rating could be lowered. That would raise interest rates and slow down the already weak economy.
A Federal Reserve survey said the economy weakened this summer.
The Dow Jones industrial average fell 199 points, or 1.6 percent, to close at 12,303 Wednesday.
About 11 stocks fell for every one that rose on the New York Stock Exchange. Volume was heavier than usual at 4.6 billion shares.