Wisconsin Elections Board executive director Kevin Kennedy estimates that turnout will be 75-percent. He says that would be the highest ever.
Besides deciding who will be president, Wisconsin voters are also deciding whether Republican Tim Michels should replace Democratic U.S. Senator Russ Feingold.
Also up for election are Wisconsin's eight U.S. House seats and much of the state Legislature.
Some light rain and drizzle was falling across much of the state when the polls opened. But it ended in the heavily populated southeastern part of the state a short time later. Gradual clearing is forecast by Tuesday night.
Temperatures ranged from the mid 30s in the north to the low and mid 40s in the south when the polls opened. They're forecast to reach highs from the mid 40s to around 50.