Democratic Sen. Herb Kohl won't be running next year, opening up a competitive race in Wisconsin. Kohl, 76, is the sixth Democrat to announce plans to retire in advance of the 2012 elections. Two Republicans have announced they won't be running. Democratic strategists say the union fight with Gov. Scott Walker has energized their base and they'll fight to keep Kohl's seat. Rep. Ron Kind and Sen. Russ Feingold are likely leading candidates. The obvious pick for Republicans is Rep. Paul Ryan, the chairman of the House Budget Committee and author of the GOP's controversial budget plan. Kohl is in his fourth term.