"I will listen carefully to my fellow Wisconsinites talk about their concerns, especially those involving their economic well-being. I will also seek their counsel on how I can best further serve my country and the state I love," Feingold wrote.
The post comes as Feingold wraps up his tenure at the State Department as a special envoy for the Great Lakes Region and the Democratic Republic of the Congo. Many Democrats are hoping he will challenge Johnson.
Johnson unseated Feingold, a liberal favorite, in 2010. Democrats see Johnson as one of the most vulnerable Republican senators running for reelection in 2016. They expect the presidential election to boost Democratic turnout in Wisconsin, helping Feingold or whoever the Democrats nominate.
Feingold has a loyal following in the state's sizable liberal base. After him, the Democratic bench drops off sharply in the state.
Senate Democrats need to gain four seat sin 2016 to win back the majority -- five if a Republican were to win the presidential election.