President Barack Obama is running unopposed on the Democratic ballot.
In Milwaukee County, mayoral races will be held in Milwaukee, Oak Creek and Wauwatosa.
Milwaukee Mayor Tom Barrett is expected to easily win re-election, even as he has declared himself to be a candidate for governor.
In Oak Creek, Alderman Steve Scaffidi is being challenged by former Alderman Mark Verhalen to succeed former Mayor Dick Bolender, who died in December.
In Wauwatosa, Pete Donegan, a former alderman who is a retired insurance executive, is vying against Kathy Ehley, executive director of the Village of Wauwatosa Business Improvement District.
Residents of Milwaukee County also will cast ballots on an advisory referendum asking for the size of the county board to be reduced.
Wisconsin voters will not be required to show photo identifications today, because a judge threw out a photo I.D. law that had been approved by the state legislature.
The polls will close at 8 p.m.