SIOUX FALLS, S.D. — Democratic U.S. Sen. Tim Johnson of South Dakota plans to retire at the end of his term, party officials said — a departure that gives Republicans another opportunity to win back control of the chamber in 2014.

Johnson, the chairman of the Senate Banking Committee, suffered a brain hemorrhage in 2006 but later returned to the Senate and won re-election in 2008.

Johnson’s retirement announcement is the sixth by Senate Democrats. Sens. Carl Levin of Michigan, Tom Harkin of Iowa, Jay Rockefeller of West Virginia and Frank Lautenberg of New Jersey also say they won’t run for re-election in 2014. Two Republican senators also have said they plan to retire.

Republicans need to add six seats to win a majority in the Senate, and South Dakota now leaps to the top of the party’s list of favorable states. Republican Mike Rounds, a popular former two-term governor, has been campaigning for the seat since last year.