Nonpartisan forecasters and opinion polls suggested a strong chance of Republicans winning a House majority and a tight race for Senate control.
Shares in healthcare, energy and defense sectors could see more volatility in the wake of the election.
"A Republican victory is seen as removing either current regulations, like in the energy sector, or potential future regulations like in the pharmaceutical sector."
The S&P 500 (.SPX) is up nearly 9% from its recent October lows, but has still posted steep losses this year on worries that aggressive monetary policy tightening would tip the U.S. economy into a recession.
Among S&P 500 sectors, energy (.SPNY) and consumer discretionary (.SPLRCD) fell the most, down 1% and 0.5%, respectively.