I didn't think that Former Sen. Russ Feingold (D-WI) would actually consider running against Gov. Scott Walker (R-WI) in 2014, but, speaking at the Wisconsin Democrats' state convention, Feingold has stated that he won't be a candidate for public office in 2014:
Feingold to WI Ds: "I don't come to you tonight as a candidate, at least not in 2013 or 2014, or 2015." Next Sen race is 2016.
Feingold, however, did not rule out a run for U.S. Senate against Ron Johnson in 2016. Feingold lost a bid for a fourth term in the Senate to Johnson in 2010.
Despite Walker's approval rating having been at or around 50% ever since the 2012 recall attempt against Walker, Wisconsin's poor economic performance (the Philadelphia Federal Reserve's index of growth indicators ranks Wisconsin second-to-last in projected economic growth) under Walker means that Walker is much more vulnerable to losing re-election than his approval rating would indicate.
However, Walker's Democratic challenger will be someone other than Russ Feingold. Two lists of potential candidates to Walker can be found here and here.