VERMILLION, S.D. (AP) -- Entering the auditorium on a motorized scooter and delivering remarks in halting speech, South Dakota Sen. Tim Johnson announced Tuesday that he would retire next year at the end of his term and acknowledged he remains limited by a 2006 health crisis that nearly killed him.
Smiling and joking at times as he made the announcement at a press conference in his hometown of Vermillion, the 66-year-old said the effects of his life-threatening brain hemorrhage had made speech and mobility increasingly difficult.
"I feel great, but I must be honest ... I appreciate my right arm and right leg aren't what they used to be, and my speech is not entirely there," Johnson said, his delivery slurred at times.
Johnson, who until recently relied on a cane to get around, has become dependent on the motorized scooter he used to enter the auditorium at the University of South Dakota where he made his remarks. He also said he hoped to spend more time with his five grandchildren and that "I think mostly it's time to go."
Johnson's departure helps solidify GOP prospects of claiming the state's first open Senate seat since 1978. Republican former Gov. Mike Rounds had announced plans last year to challenge Johnson, the chairman of the Senate Banking Committee, and has rallied much of the state and national party establishment behind him.
In a sign Republicans are not yet lock-step behind Rounds, a Washington-based political action committee released a statement Tuesday criticizing Rounds' spending during two terms as governor, and calling for a more conservative candidate to run for Johnson's seat. The same fractures between tea party-aligned and more moderate Republicans cost the GOP seats that were seen as winnable in 2010 and 2012.
On the Democratic side, long-rumored possible successors including Johnson's son, South Dakota's U.S. Attorney Brendan Johnson, and former U.S. Rep. Stephanie Herseth Sandlin.
"I've talked to Brendan now and then, but I'm not leaning on him to run or not run," Johnson told The Associated Press after the event. "I gather he is undecided."
Brendan Johnson declined to comment on his political plans Tuesday. He has avoided public statements about the race, but has had conversations with party activists in the state, and advisers and potential donors outside South Dakota.
Herseth Sandlin, 42, is general counsel for Raven Industries, Corp., a position she began last year after returning to South Dakota from Washington, D.C. She had worked as a lawyer in Washington after losing re-election to a fourth U.S. House term in 2010.
"While I appreciate the encouragement I've received I haven't focused on the future political opportunities," she told The AP.
Although both prospects have political networks to tap, both also face potential liabilities.
The younger Johnson, 37, has never held political office and would face questions about his father's involvement in the confirmation process to his federal post. Herseth Sandlin has taken some positions at odds with some South Dakota Democratic activists, including opposing the 2010 Affordable Care Act, which could hurt in a potential Democratic primary.
Republicans too could face ideological tension. U.S. Rep. Kristi Noem, elected in 2010, has been courted to challenge Rounds by South Dakota conservatives. While Noem has shown little movement toward investigating a 2014 Senate campaign, aides said Tuesday: "She hasn't ruled anything in or out."
Nationally, Republicans said by stepping aside, the politically resilient Johnson, who also retains a $1.2 million campaign war chest, gives the GOP its best chance to gain a seat in its quest for the majority.
"I believe South Dakota moves into the top slot as the most likely Republican pickup," said Greg Strimple, a Republican pollster and past consultant to the National Republican Senatorial Committee.
Johnson, chairman of the Senate Banking Committee, joins Democratic Sens. Carl Levin of Michigan, Tom Harkin of Iowa, Jay Rockefeller of West Virginia, and Frank Lautenberg of New Jersey as seasoned and influential Democrats departing the chamber, where Republicans need to gain six seats to take control. Two Republican senators have announced their retirements, both in Republican-performing states Georgia and Nebraska.
South Dakota was carried by Republican presidential candidate Mitt Romney last year, adding urgency for Democrats hoping to keep their majority. Republicans must gain six seats to retake the chamber in 2014.
With his wife Barbara at his side, Sen. Johnson remained standing, leaning on his stronger left hand at the podium, taking questions from reporters then posing for pictures and shaking hands with supporters -- reaching out with his left -- for an hour.
"I look forward to serving the remaining two years as the country is facing difficult times on many fronts and I will work every day to find a bipartisan solution to these challenges," he said.
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