Trading on the nation's oldest exchange that enables investors to place online bets on presidential candidates shifted sharply late yesterday afternoon in favor of Sen. John F. Kerry.

The Democrat's stock, in a "winner-take-all" contest run by the University of Iowa's Iowa Electronic Markets hit 68 cents per share at 4:45 p.m. EST, vs. 32 cents for President Bush. The prices mean traders were giving Kerry a 68 percent chance of winning over the Republican incumbent. Just 24 hours before, Bush's stock was ahead, with traders giving him slightly better than a 50 percent chance of winning.

For most of the campaign, Bush had been either leading his rival or running neck and neck. But the past couple of days have brought a huge increase in trading volume, sending the number of shares changing hands to roughly 40,000, or 10 times normal on Monday, said George Neumann, co-founder and director of IEM.

Neumann said that in the past several days, daytime trading has tended to give Kerry a lift that sometimes decreases overnight.

He said IEM sponsors were at a loss to explain the pattern, but there was some speculation that it might be a "student effect" with students dominating trading during the day. The IEM, founded in 1988 as a research tool for students and faculty, has since opened its political markets to outside traders.

Trading in the winner-take-all contest is technically supposed to finish this morning, with investors who correctly chose the winner of the nation's popular vote cashing in at $1 on each share they hold.

However, should recounts and or other vote-tallying delays frustrate this year's presidential election as they did four years ago, the IEM will continue trading. "If need be we'll keep [the market] open as long as the outcome is in doubt," Neumann said.