"Senator Kohl has served the state with honor. He will be remembered for his advocacy for our state's dairy farmers, his work on behalf of children and his keen understanding of our state's business community," Feingold said in a statement. "I sincerely wish him all the best."
"I was surprised by Senator Kohl's announcement and want to take some time over the next few days to discuss this news with my family and supporters before making any decision about how I'm best able to serve my employers in the First Congressional District, our state and nation," said Ryan.
Other Republican candidates include: Wisconsin Attorney General JB Van Hollen, and former Feingold challengers Tim Michels and Mark Neumann.
"We have not committed fully to running at this point," Neumann told Hotline On Call Friday morning. "If we were to get in to the race, I was looking forward to running against [Kohl's] record," he added.
Neumann said he does not believe that Kohl's retirement significantly changes his decision making-process or timeline for making a decision. Neumann told Hotline On Call in April that before he makes any decision, he wants to know what Ryan's plans are.
Democratic officials have mentioned Reps. Ron Kind (D-Wis.),Tammy Baldwin (D-Wis.) and Milwaukee Mayor Tom Barrett (D) as possible contenders.
Updated at 12:37 p.m.