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.
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.
Former Democratic Rep. Stephanie Herseth Sandlin has also been mentioned as possible candidate. She scored higher approval numbers than the senator in the PPP poll.
An official with the South Dakota Democratic Party predicted that Sandlin and Johnson’s son would not compete against each other in the primary.
The official, who did not speak for attribution because the senator has not publicly announced a decision, said one of the candidates would likely forgo a Senate race and run for either the House or the governor’s office in 2014.
“I don’t think we’ll have a primary,” the official said, while declining to speculate on which candidate would enter the Senate race.
Johnson, who is chairman of the Senate Banking Committee, was first elected in 1996 after serving eight years in the House. He suffered a brain hemorrhage in 2006, which kept him out of Congress for a year.
In a sign he was not angling for reelection, Johnson voted in favor of the Democrats’ budget last week. Four other Democratic senators up for reelection from red states voted "no" on the budget, which contains nearly $1 trillion in tax increases.
Johnson’s retirement adds to the challenges facing Democrats as they try to hold onto the Senate majority.
Four other veteran Democratic senators have already announced they won’t seek reelection next year: Sens. Tom Harkin (Iowa), Frank Lautenberg (N.J.), Carl Levin (Mich.) and Jay Rockefeller (D-W.Va.).
The Democratic Senatorial Campaign Committee declined to comment until Johnson announces his decision on Tuesday.
On the Republican side, Sens. Saxby Chambliss (Ga.) and Mike Johanns (Neb.) have announced they will not seek reelection, but both of those seats are likely to remain in the Republican column.
—Last updated at 6:18 p.m.