Apparently, State Senator Rick Gudex (R-Fond du Lac) has decided he can't beat Mark Harris in 2016

Winnebago County Executive Mark Harris, a Democrat, announced his candidacy for the 18th Senate District seat in September.

In November 2012, Rick Gudex beat the incumbent Jessica King (D-Oshkosh) by 600 votes -- 43,079 (50.3 %) to 42,479 (49.6%)

As quoted in State Senator Rick Gudex Announces He Will Not Seek Re-election.  (Press release, 11/30/2015)

Other Gudex posts:
2016 match-up in 18th Wisconsin State Senate District.  (9/19/2015)
Rick Gudex's case of selective amnesia.  (8/29/2014)
Once bitten, twice bitten in Wisconsin's 18th Senate district.  (5/7/2013)
Bought and paid for.  (5/3/2013)
State Senator Rick Gudex summarizes the Governor's budget priorities.  (1/22/2013)