South Dakota Sen. Tim Johnson was expected to announce his retirement Tuesday, making the state the fifth where Democrats will have to defend a seat without an incumbent seeking re-election. The decision opens up a 2014 race that Republicans had already labeled as a top target to grab a seat.
Johnson, the chairman of the Senate Banking Committee, is expected to say Tuesday afternoon that he will not seek a fourth term in the Senate next year.
Former Gov. Mike Rounds, a Republican, had announced plans last year to challenge Johnson, who still is coping with the constraints of a 2006 brain hemorrhage that left his speech impaired and limited his mobility.
By stepping aside, the 66-year-old Johnson, who had never lost an election, leaves South Dakota Democrats searching their youthful prospects for a potential successor.
"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.
Aware that Johnson might decide to retire, Democrats in South Dakota and nationally have discussed possible successors on the ticket, chiefly Johnson's son Brendan Johnson and former U.S. Rep. Stephanie Herseth Sandlin.
But 37-year-old Brendan Johnson, who was appointed U.S. attorney for South Dakota in 2009, has never held elected office and faced questions about his father's involvement in the confirmation process.
Like the younger Johnson, Herseth Sandlin is a well-known name in the state and has a fundraising base. But the 42-year-old could face problems in a Democratic primary in light of her opposition to the 2010 Affordable Care Act, a position that is out of step with a majority of party loyalists.
Johnson was scheduled to speak Tuesday at the University of South Dakota in Vermillion.
For weeks, Johnson's longtime aides and allies have suggested he would step down in 2014, despite his return to the Senate after the life-threatening hemorrhage. His recovery has been significant, though his speech remains impaired and he sometimes uses a motorized scooter.
Johnson's re-election in 2008 after the brain injury sealed his reputation for resilience. But long before, he had established a profile as a loyal Democrat but with an independent streak that made him a formidable candidate. He won re-election to the Senate against the popular Republican U.S. Rep. John Thune, now South Dakota's junior senator.
Johnson has sided with Democrats on key issues such as the 2010 Affordable Care Act. He also has been an environmental advocate. But last week he voted for a resolution supporting the Keystone XL oil pipeline, which is mapped to cross South Dakota and fiercely opposed by environmentalists.
He also has $1.2 million in his re-election campaign account, a healthy nest egg for a state where television advertising is relatively inexpensive. He retains a robust fundraising network, thanks to his deep-pocketed committee connections.
Despite those advantages, Johnson 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. Among those states, West Virginia was carried by Republican presidential candidate Mitt Romney last year.
Two Republican senators have announced their retirements, both in Republican-performing states Georgia and Nebraska.
South Dakota, a reliably independent state just a decade ago, has trended sharply Republican in the past decade. Where Democrats once boasted two influential Democratic senators, Republicans now control the Legislature, governorship, the lone U.S. House seat and the other Senate seat.
But Democrats dismissed 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.
"I reject the idea that somehow the Republicans have a lock on this state," South Dakota Democratic Party Chairman Ben Nesselhuf said. "By no means is this an impossible task, or even improbable."
Devotees say Johnson's personality — reserved and contemplative — has been the key to the respect and influence he has amassed.
Bernie Hunhoff, minority leader in the South Dakota House, described Johnson as a pioneering advocate for women's and children's issues during his early days in the state Legislature. Johnson set the standard for Democrats, by staying true to progressive principles, while also reaching out across the electorate in a politically diverse state, said Hunhoff.
"He's more progressive than the average voter, but they obviously set that aside because they liked what they saw," Hunhoff said. "I think you'd describe Tim as a typical South Dakotan — quiet, stoic, practical — a good neighbor."
Despite contentious congressional and campaign battles, Johnson has remained "a gentleman," known more for thinking than talking, former South Dakota Democratic Party Chairwoman Judy Olson Duhamel said. "He's earned the respect of everybody he's ever worked with or for," she said.
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 announce his political plans Tuesday in Vermillion.
Brokaw reported from Pierre, S.D.; David Espo contributed from Washington.