Dem Senator Tim Johnson Expected to Announce Retirement

Likely Republican pickup. Romney won the state of South Dakota 57%-39% over Obama in 2012.

There is a growing list of Dem retirements plus the fact that Democrats have 21 seats to defend in 2014 vs. 14 for Republicans

Senate Democrats are expecting their colleague Sen. Tim Johnson to announce his retirement at an event in his home state of South Dakota Tuesday, according to a senior Democratic congressional source.
The event is scheduled to take place at Johnson's alma mater, the University of South Dakota. A press release said the event would be "regarding the 2014 election." Johnson was first elected to the U.S. Senate in 1996 and served in the upper chamber for three terms.  
In 2006, a brain hemorrhage almost killed him, but he recovered and went on to run for and win re-election in 2008. While lingering effects of the brain hemorrhage have caused Johnson's speech to be slurred, he's remained active on the Senate Appropriations Committee and the Banking Committee, which he chairs.  
Johnson will become the fifth Democratic senator to announce his retirement in 2014. Republicans from Georgia and Nebraska have also announced they won't run for re-election.