The stock market sustained more losses Tuesday, made all the more disheartening by a sell-off in the closing minutes. The Nasdaq tumbled 1.3% and the S&P 500 0.9%. Both started bouncing back in the afternoon, in what was shaping up as a bullish reversal. But selling washed over the exchanges in the final 20 minutes of trading. Thus, the best attempt so far to stop the market slide collapsed in a flash of selling.