The Dow Jones industrial average plunged 328 points to 12,916 at midday Wednesday. The Dow was on track for its worst decline in a year.
The Standard & Poor's 500 index lost 36 points to 1,391. The Nasdaq composite fell 78 points to 2,933.
Energy companies and banks took big losses. Both industries presumably would have faced lighter regulation if Mitt Romney had won.
Stocks seen as benefiting from President Barack Obama's decisive win rose. They included hospitals, free of the threat that Romney would have rolled back Obama's health care law.