NEW YORK (CBSNewYork/AP) — A warning from Standard & Poor’s that the agency might lower its rating on U.S. government debt sent stocks on their worst slide in a month.

S&P said Monday that there was a 33 percent chance it would lower the country’s credit rating from AAA in the next two years if Washington failed to pare the country’s debts.

 The Dow Jones industrial average and the S&P 500 index both had their steepest falls since March 16. The Dow fell 140 points, or 1.1 percent, to close at 12,202. The S&P 500 fell 15, or 1.1 percent, to 1,305. The Nasdaq composite fell 29, or 1 percent, to 2,735.

Four stocks fell for every one that rose on the New York Stock Exchange. Trading volume was 4.6 billion shares.