Republican state Sen. Rich Zipperer of Pewaukee is leaving the Senate to join Gov. Scott Walker's administration as deputy chief of staff.
Walker announced Wednesday that Zipperer will begin the new job on Aug. 6, leaving Republicans with just 15 members in the Senate. Democrats have 17.
The 38-year-old Zipperer was elected to the Senate in 2010 after serving two terms in the Assembly.
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.
Rep. Paul Farrow, a Pewaukee Republican elected to the Assembly in 2010, announced hours after Zipperer's appointment was made public that he would run for the Senate seat.