South Dakota Sen. Tim Johnson will not seek re-election in 2014, vacating what was already one of the Democrats’ most vulnerable seats. Johnson announced his retirement Tuesday at his alma mater, the University of South Dakota, in his hometown of Vermillion. "I will be 68 years old at the end of this term, and it is time for me to say goodbye," Johnson said at a press conference. He thanked Democrats, Republicans and independents for supporting him in 12 straight general elections since 1978. Senate Republicans already indicated they will target the seat, which is one of the party's top pickup opportunities. The GOP already boasts a top-tier candidate in the race, former two-term Gov. Mike Rounds. However, the GOP primary field could grow as other Republicans, including Rep. Kristi Noem, continue to consider the race. And the Senate Conservatives Fund announced Tuesday that it is seeking an alternative to Rounds, the first outside group to indicate it intends to play in the primary. (more…)