New Delhi, Nov. 6 -- The fears had shadowed Wall Street off and on for months. Then, all at once, everyone rushed for the exits.

The stock-market rout on Thursday, which began soon after the opening bell on the New York Stock Exchange, worsened as the day wore on, pulling down everything from high-flying chipmakers like Nvidia Corp. to defensive fallbacks like Walmart Inc.

But hardest hit were the companies at the heart of the artificial-intelligence boom that earlier this year sent the stock market to one record high after another - and sowed worries that it was a bubble bound to burst.

All it took to send them tumbling on Thursday was the prospect that Democrats' sweeping victory in the Congressional elections may herald a political ...