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.

Leaving aside the issue of this particular year’s unusual imbalance of seats up for election, in general, for the long-term good of the Democratic Party, this is basically the optimum time for any of its members to retire.

2012 is a year with a Presidential election, which means a higher turnout among groups that lean Democratic over a congressional midterm year, and significantly higher turnout over special elections. Not only is 2012 a Presidential election year, but it is more likely than not a Democrat will win the Presidency. As basic conditions go, that is about as favorable as it gets for a Democrat running in an open race.

Senators like Kohl, age 79, and Daniel Akaka (HI), age 86, are (one way or another) going to need to stop being senators eventually. For the long-term prospects of the party, of all the moments they could leave office, choosing not to run again and creating an open race in a Presidential election is the way that is least disruptive to the party.

Even if the recent retirements create some problems for the Democrats now, this is, in general, a very good time for Senate Democrats up for re-election to step aside.