Associated Press
Tim Johnson

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 Monday — a departure that gives Republicans a prime opportunity to pick up a seat as they attempt 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. He has recovered significantly, though sometimes uses a motorized scooter.

The Democratic officials who described Johnson's plans spoke to The Associated Press on condition of anonymity. They said they were not authorized to pre-empt a formal announcement expected today at the University of South Dakota in Vermillion.

Johnson's retirement announcement is the fifth 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 have also said they plan to retire.

Republicans must gain six seats to win a majority in the Senate, and South Dakota now leaps to the top of the party's list of most favorable states. Republican Mike Rounds, a popular former two-term governor, has been campaigning for the seat since last year, though he declined to comment Monday on Johnson's retirement.

Johnson is viewed as independent minded and reserved, having voted against the resolution to allow military action in Iraq but supporting the Keystone XL Pipeline. He also has about $1.2 million in his campaign account, a healthy nest egg for a state where advertising is relatively inexpensive, and a deep-pocketed fundraising network.

While those votes and fundraising make the former congressman, who has never lost an election, a formidable opponent, South Dakota's GOP-trending electorate could complicate the next election. His last term also has been physically demanding for the 66-year-old Johnson, whose speech remains compromised.

But Democrats rejected the notion that Johnson's retirement opens the door for a GOP senator. In last November's election, some Republican Senate candidates who appeared to be the heavy favorites ended up losing to Democratic rivals — including Rick Berg, who lost to Heidi Heitkamp in neighboring North Dakota.

South Dakota Democratic Chairman Ben Nesselhuf noted Democrats' successes over the past 30 years, including former Senate Democratic Leader Tom Daschle.

"I reject the idea that somehow the Republicans has a lock on this state," Nesselhuf said. "By no means is this an impossible task, or even improbable."

Attempts by The Associated Press to reach Johnson through his top Senate staff were unsuccessful. Johnson aides would not confirm the retirement but said the senator would hold a news conference on his political future at University of South Dakota in Vermillion Tuesday afternoon.

Aware that Johnson might decide to retire, Democrats in South Dakota and nationally have discussed possible successors on the ticket, including Johnson's son Brendan, South Dakota's U.S. attorney.