Republican state Rep. Chris Kapenga said he will run for the state Senate in the Milwaukee-area seat that will be vacated by Sen. Rich Zipperer of Pewaukee, a fellow Republican.
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, another 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 November 6 presidential election.
With Zipperer's departure, Democrats will hold a 17-15 majority in the Senate.
The soon-to-be open Senate seat has been in Republican hands for decades.