His retirement would leave a vacant seat in a conservative-leaning state that could be difficult for Democrats to defend as they try to protect their majority in the Senate.

Political analysts expect Johnson’s son, Brendan Johnson, who is South Dakota’s U.S. attorney, to emerge as a potential Democratic candidate in the 2014 election. The younger Johnson has not announced any formal plans to seek the Senate seat.

Former Representative Stephanie Herseth Sandlin, who lost a bid for reelection in 2010, is another leading choice to run if the incumbent senator retires.