According to the Wisconsin State Journal, Justice Prosser is retiring in July:
State Supreme Court Justice David Prosser, a conservative whose opinions helped solidify a series of controversial changes in state law in recent years but was also known for his physical confrontation with a colleague, announced Wednesday he will retire in July.
Prosser’s retirement comes after 18 years on the court and five years before his current 10-year term ends. Gov. Scott Walker will need to appoint a justice to replace Prosser, part of the court’s 5-2 right-leaning majority.
State law requires Walker’s appointee to serve until the next spring election that does not have a Supreme Court justice on the ballot, according to Walker spokesman Jack Jablonski, which is 2020.