Democratic state Sen. John Lehman of Racine plans to give up the seat he won in a 2012 recall election.
Lehman on Monday became the first Democrat to announce plans to run for Lt. Gov.
If he would have again vied for Senate, the race might have turned into yet another rematch between Lehman and Republican Van Wanggaard.
The two squared off in 2010 with Wanggaard winning; then Lehman narrowly won last year's recall race.
Wanggaard is again seeking the Senate seat in 2014.