Zipperer is leaving the Senate next month to join Gov. Scott Walker’s administration as deputy chief of staff.
Also running for Zipperer’s old seat is Rep. Paul Farrow, a Pewaukee Republican.
Both Kapenga, who is from Delafield, and Farrow were elected in 2010.
Under state law, the earliest Walker could call a special election to fill the seat would be early December, which could allow a primary, if necessary, to be on the same day as the Nov. 6 presidential election.
The open Senate seat has been in Republican hands for decades.