The only Wisconsin Republican to vote against Act 10 and the new state mining law will retire after serving 31 years in the Legislature.
Sen. Dale Schultz was facing a challenge in this fall's elections from Rep. Howard Marklein. Marklein said he decided to try to oust Schultz because the incumbent rejected Act 10, which weakened public unions, and the mining law that hastened state procedures for approving a proposed iron ore mine in northern Wisconsin.
Schultz has been considered a moderate Republican, Marklein, a conservative.
In announcing his retirement, Schultz said he would not endorse Marklein, calling him "out of step" with constituents' views. The district is located in southwestern Wisconsin.
One Democrat is running for the Senate seat - Ernie Wittwer.