Sen. Tim Johnson is latest Democrat set to retire WASHINGTON - Democrats' efforts to maintain a majority in the U.S. Senate after next year's midterm election were thrown into further doubt Tuesday when Sen. Tim Johnson of South Dakota became the fifth senior member of the party to announce his retirement. Johnson, who chairs the Senate Banking Committee, said he would not seek a fourth term. He suffered a brain hemorrhage in 2006 and, despite extensive rehabilitation, relies on a wheelchair to travel the halls in Congress. "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 at the University of South Dakota. Two weeks ago, Sen. Carl Levin of Michigan said he would step down after his sixth term. Fellow Democrats John D. Rockefeller IV of West Virginia, Tom Harkin of Iowa and Frank R. Lautenberg of New Jersey also won't seek reelection. As a result, at least five of 21 Democratic Senate seats on the ballot in 2014 will not include an incumbent. Only two Republican senators have announced retirement plans: Saxby Chambliss of Georgia and Mike Johanns of Nebraska. Democrats say the early flurry of decisions is deliberate: Party leaders pressed lawmakers to make their plans known early so potential Democratic successors could prepare campaigns to hold the seats. The last two midterm cycles - in 2006 and 2010 - saw decisive swings against the party in the White House. But analysts say it is too early to know whether Democrats will face major head winds in 2014. Senators running for reelection next year won their seats in 2008, when Barack Obama first won the presidency in a historic campaign. The race put Democrats one vote short of a filibuster-proof 60-vote supermajority in the Senate, which they briefly held after then-Sen. Arlen Specter of Pennsylvania switched parties. After losing seats in the 2010 midterm, the party gained again last year and holds a 53-45 advantage over the Republican Party. Two independents caucus with the majority. If they continue to do so, Republicans would need to pick up six seats next year to regain a majority. Both parties temper expectations at this point, pointing to a major determinant of recent Senate races: the caliber of Republican candidates. "It's incumbent upon us to take advantage of a positive playing field and to recruit consensus candidates ... who can unite the local party running in key states," said Brad Dayspring, spokesman for the National Republican Senatorial Committee. Democrats must defend seats in seven states that GOP presidential nominee Mitt Romney carried in 2012, all but one by double-digits. But Democrats say their incumbents remain popular in most of those states, limiting Republican chances. Republicans "basically have to run the table in those seats. That's a hard thing for Republicans to do given the turmoil in their party right now," said Matt Canter of the Democratic Senatorial Campaign Committee. "The Republican establishment claims they are going to do a better job managing the tea party and handpicking mainstream candidates that can win, but they don't seem to have any good strategy to do that," he added. Democrats say they hope they can make two Republican-held seats competitive: In Georgia, where no incumbent is running and Republicans are likely to face a bruising multi-candidate primary; and in Kentucky, where Senate Minority Leader Mitch McConnell may be vulnerable. In South Dakota, Republicans have a top-caliber recruit in former Gov. Mike Rounds. But the Senate Conservatives Fund, a conservative political action committee affiliated with former Sen. Jim DeMint of South Carolina, issued a statement Tuesday saying it would consider supporting a "true conservative" to challenge Rounds. Democrats see two potential candidates: Brendan Johnson, the incumbent's son and South Dakota's U.S. attorney; and former Rep. Stephanie Herseth Sandlin, who lost her seat in the 2010 midterm that President Obama dubbed a "shellacking."