Democratic Sen. Tim Johnson of South Dakota has reportedly decided not to seek re-election next year, which will spark a wide-open battle for the seat in a state President Obama lost last year. ...

Johnson, chairman of the Senate Banking Committee, is set to make a formal announcement about his political future Tuesday at the University of South Dakota, his alma mater.

An open seat in South Dakota would be coveted by both parties, especially Republicans who need a six seats to win the majority. Johnson would be the fifth Democrat to not seek re-election in 2014. Two Republicans have already announced their retirements.