Dane County District Attorney Ismael Ozanne, Jefferson County District Attorney Susan Happ and state Rep. Jon Richards of Milwaukee are all vying for the Democratic nomination. Waukesha County District Attorney Brad Schimel is the only Republican in the race.
Thomas A. Nelson, Sr., filed to run as a Libertarian.
The Government Accountability Board is scheduled to meet on June 10 to certify the list of candidates for the Aug. 12 primary.
The winners will face each other in the Nov. 4 general election.
The seat is open after Republican Attorney General J.B. Van Hollen decided against seeking a third term.