Clark announced Friday that he will retire at the end of the year.
The 54-year-old Clark was first elected in 2008. In 2011 he ran against Republican Sen. Luther Olsen in a recall election but lost.
Clark is one of 39 Democrats in the Assembly. Republicans have 60 members.
Clark's district covers an area just north of Madison and includes Sauk, Columbia, Marquette, Dane, Adams and Iowa counties.