Sen. Tim Johnson (D-S.D.) will not seek reelection in 2014, The Hill has confirmed, putting another red-state seat up for grabs in the battle for control of the Senate.

Johnson, who is serving his third term, will announce the news on Tuesday at the University of South Dakota, his former school. Johnson will be the fifth Senate Democrat to retire this election cycle.

The decision gives Republicans another prime pickup opportunity as they work to win back control of the Senate.

Republicans need to gain six seats to flip control of the upper chamber in 2014. Mitt Romney carried South Dakota with 57 percent of the vote in the 2012 presidential election.

The National Republican Senatorial Committee said the seat would have switched parties whether Johnson retired or not, judging by Romney's big win in the state.

"South Dakota voters overwhelmingly rejected the Democratic agenda by 20 points in 2012 and is a prime pick up opportunity for Republicans regardless of whose name Democrats put on the ballot," said Brad Dayspring, communications director for the NRSC.

The GOP already has a strong candidate in former Gov. Mike Rounds, who announced his candidacy last year and leads all potential Democratic opponents, according to a poll from the Democratic-leaning Public Policy Polling released last week.

But the eventual Republican nominee could face a stiff challenge from another member of the Johnson clan: U.S. attorney for South Dakota Brendan Johnson, who has not ruled out a run for his father's seat.

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