The announcement by Sen. Herb Kohl (WI) that he will not be seeking re-election makes him the sixth member of the Democratic caucus that has chosen not to run again this year. With Democrats forced to defend 23 of the 33 senate seats up in 2012, the loss of the incumbent advantage in another race is a minor setback for the party as it hopes to retain control of the Senate in 2012.

While winning the minimum 25 seats required to regain the majority would be difficult, looking at the historic record, it is possible.