WASHINGTON - South Dakota Democrat Tim Johnson's decision to retire from the Senate in 2014 is another jolt to his Senate Democratic colleagues.
Four other Senate Democrats have decided to exit the "world's greatest club" in two years.
They include Michigan's Carl Levin, New Jersey's Frank Lautenberg, Iowa's Tom Harkin and West Virginia's Jay Rockefeller.
Democrats say they think they've got a good chance of holding Michigan and New Jersey, but Republicans could offer strong challenges in Iowa and West Virginia.
Republicans are eyeing South Dakota as a special prize.
Possible candidates on the Democratic side in 2014 include Johnson's son Brendan and former Rep. Stephanie Herseth Sandlin, who lost her seat in the 2010 tea party wave.
On the GOP side, former Republican Gov. Mike Rounds had already announced plans to challenge Johnson.
Conservative critics charge Rounds is a big spender and are looking for some one who is more ideologically acceptable.
Republicans need six seats to take over the Senate.
Johnson's retirement has given them new hope for a sweep.
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