He isn't committing to a formal announcement, but on Thursday Sen. Tim Johnson (D-S.D.) sounded like a man running for reelection. 

"As in past campaigns, I will make my formal announcement later next year," Johnson said in a statement. "But I feel great, still have work to do, and I fully intend to put together a winning campaign in the weeks and months ahead."  

Johnson was responding to former Republican governor Mike Rounds's announcement that he would run for the seat. Rounds is a top GOP recruit, and Johnson has been considered a likely 2014 retiree.

"I consider Mike a friend, and I welcome him to the race," the Democratic senator said in his statement. He praised Rounds for refusing to commit to Grover Norquist's anti-tax pledge. "While I'm sure that Mike and I will have some policy disagreements – which is to be expected – I am more than willing to give credit where it is due."