Democratic strategists insisted that they will field a top-tier candidate in the open seat, noting that the fight between Gov. Scott Walker (R) and national unions over collective bargaining in the state earlier this year has energized the party. Rep. Ron Kind and well as former Sen. Russ Feingold are sure to be leading candidates.

For Republicans, the obvious name is Rep. Paul Ryan, the architect of House Republicans’ controversial budget plan. It’s not clear whether Ryan wants to leave his perch as chairman of the House Budget Commtitee to make a statewide run, however.