Dire economic data knocked stocks sharply lower Wednesday, with the S&P 500 posting its worst single-day percentage decline since Black Monday 1987, as investors braced themselves for an ugly recession.

The session's drop rekindled debate on Wall Street about whether last week's lows will hold up. Increasingly, it seems the record 936-point gain registered by the Dow Jones Industrial Average Monday wasn't enough to put the market on sure footing.