The market was up for much of the day but started to sink in the last half-hour of trading. Analysts said that was because of investors' skepticism that a resolution could be reached in time to raise the country's borrowing limit.
The Dow Jones industrial average fell 62 points, or 0.5 percent, to close at 12,240. The index had been up as many 82 points earlier.
The Standard & Poor's 500 fell 4, or 0.3 percent, to close at 1,301. The Nasdaq rose 1 to 2,766.
Nearly two stocks fell for every one that rose on the New York Stock Exchange. Volume was relatively heavy at 4.4 billion shares.