(CNN) – Sen. Tim Johnson says it's time for him to say so long.
The three term Democrat from South Dakota announced Tuesday that he won't run for re-election next year.
"I will be 68 years old at the end of this term and it is time for me to say goodbye," said Johnson, who was accompanied by his wife Barbara at an event at his alma mater, the University of South Dakota.
"I'm honored and humbled that the people of South Dakota, Democrats, Republicans and independents, have allowed me to represent you in the state legislature, the United States House of Representatives and the U.S. Senate," added Johnson.
Johnson was first elected to the U.S. Senate in 1996, winning re-election in 2002. Four years later a brain hemorrhage almost killed him, but he recovered and went on to run for and win re-election in 2008.
While lingering effects of the brain hemorrhage have caused Johnson's speech to be slurred, he's remained active on the Senate Appropriations Committee and the Banking Committee, which he chairs. In his remarks, Johnson thanked South Dakotans for helping him in his recovery, saying "your patience and prayers enabled me to recover from a life threatening brain injury."
Asked if his health was behind his decision not to run again, Johnson said "I feel great, but I must be honest. I appreciate that my right arm and right leg are not what they used to be and my speech is not entirely there. But I think mostly that it's time to go."
If he had made a bid for another term in the Senate, Johnson would have faced a tough go of it, especially in a state that the GOP carried by 18 percentage points in last November's presidential election. But asked if he did run again, could he win, Johnson said to applause that "I've never been beaten."
Johnson becomes the fifth Democratic senator to announce this year that they won't run for re-election in 2014, joining Jay Rockefeller of West Virginia, Tom Harkin of Iowa, Frank Lautenberg of New Jersey, and Carl Levin of Michigan. Two Republican senators, Saxby Chambliss of Georgia and Mike Johanns of Nebraska, are also not making bids next year for another term in office.
The Democrats control the Senate 55-45 (with two independents caucusing with the party), but as of now they are defending 21 of the 35 seats up for grabs in November 2014.
"South Dakota is about as close to a must-win as it gets for Republicans if they want to take back the majority next year. It's hard to imagine a scenario where Republicans net six Senate seats and lose South Dakota," says Nathan Gonzales, deputy editor of the non-partisan Rothenberg Political Report.
"It's a prime pick up opportunity for Republicans," says Sen. Terry Moran of Kansas, chairman of the National Republican Senatorial Committee. "When given the choice between a conservative vision to grow the economy, decrease overbearing regulations, and provide opportunity to young people and the liberal Democratic agenda of higher taxes, more regulations, and fewer jobs, we have no doubt voters in South Dakota will overwhelmingly choose the former."
Senator Michael Bennet of Colorado, chairman of the Democratic Senatorial Campaign Committee, praised Johnson, saying that he "has been a tireless advocate for South Dakotans and for farm communities across the country. He will be greatly missed in the US Senate."
Bennet also thanked Johnson for making his announcement more than a year and a half before the midterm elections, saying "I appreciate that Senator Johnson has made this decision so early in the cycle, giving Democrats the opportunity to build a winning ticket next year. The DSCC will devote all of the resources necessary to win this seat next year."
Former two-term Republican Gov. Mike Rounds announced last year that he would make a bid for the Senate seat. But Rounds is not beloved by some conservatives, who are encouraging Rep. Kristi Noem to consider a run for the Senate.
On the Democratic side, U.S. Attorney Brendan Johnson, the senator's son, and former Rep. Stephanie Herseth Sandlin, are eyeing bids.
Brendan Johnson was not at his father's retirement announcement. Asked about his son's possibly candidacy, Johnson said "I think that there are several good candidates out there, and you'll have to ask Brendan about that."
After 36 years in politics, Johnson said not facing re-election will be strange but "I'm certain I can get over it."