US stocks tumbled Tuesday, sending the Standard & Poor's 500 index back below the key 1200 level following credit-rating downgrades from S&P on Greece and Portugal.

The Dow Jones Industrial Average dropped 128 points, or 1.2%, at 11079 in recent trading. Alcoa was the measure's worst performer with a drop of 3.2%. Caterpillar, which is particularly sensitive to the global economy, was also weak with a drop of 3.1%. Chevron was off 1.8% as crude-oil futures fell.