(AP Photo/Susan Walsh)
The announcement by Democratic Sen. Herb Kohl that he won't seek a fifth term has set the Democrats further back in their hopes of keeping the Senate during next year's elections. Kohl is the fifth Democrat to retire after next year, or sixth if you count independent Connecticut Sen. Joe Lieberman, who caucuses with the Democrats. With Democrats defending 23 seats, and a short list of Democratic incumbents running in conservative and swing states, Republicans are seeing more and more opportunities to pick up some seats. Here are the nine most vulnerable Democratic seats, in alphabetical order.