Two people with knowledge of his decision said Johnson will not run again -- a decision that was widely expected. The news was first reported by Reuters. A Johnson adviser declined to confirm his retirement.
Johnson survived a life-threatening brain hemorrhage in 2006, returning to the Senate in late 2007 and easily winning reelection a year later. The 66-year-old senator is still partially paralyzed; he sometimes uses a motorized scooter to get around and his speech is occasionally slurred. But he has said that his health is not an issue.
"I feel great, still have work to do, and I fully intend to put together a winning campaign in the weeks and months ahead," Johnson told The Post late last fall.
South Dakota will now have an open Senate seat for the first time since Sen. Jim Abourezk (D) retired in 1978. Johnson took his seat in 1996, defeating incumbent Sen. Larry Pressler (R) in a competitive race and narrowly holding his seat in 2002 against then-Rep. John Thune (R).
Republican former governor Mike Rounds announced last year that he will run, giving the GOP a top-tier candidate in the increasingly conservative state. Freshman Rep. Kristi Noem (R) could challenge Rounds in the primary, though.
With Johnson out, his son, U.S. Attorney Brendan Johnson, is a possible Democratic contender, as is former Rep. Stephanie Herseth Sandlin.
Including Johnson, five Democratic senators and two Republican senators have announced their plans to retire in 2014.
Updated at 5:30 p.m. Chris Cillizza contributed to this report.
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