South Dakota Sen. Tim Johnson will not seek re-election in 2014, vacating what was already one of the Democrats’ most vulnerable seats.

Johnson announced his retirement Tuesday at his alma mater, the University of South Dakota, in his hometown of Vermillion.

“I will be 68 years old at the end of this term, and it is time for me to say goodbye,” Johnson said at a press conference. He thanked Democrats, Republicans and independents for supporting him in 12 straight general elections since 1978.

Senate Republicans already indicated they will target the seat, which is one of the party’s top pickup opportunities. The GOP already boasts a top-tier candidate in the race, former two-term Gov. Mike Rounds.

However, the GOP primary field could grow as other Republicans, including Rep. Kristi Noem, continue to consider the race. And the Senate Conservatives Fund announced Tuesday that it is seeking an alternative to Rounds, the first outside group to indicate it intends to play in the primary.

The South Dakota seat will be one the toughest for Democrats to hold in 2014, but party officials say they expect to compete there.

Despite the state’s GOP lean — President Barack Obama received just 40 percent of the vote there in 2012 — Democrats have a bench of potential candidates. They include former Rep. Stephanie Herseth Sandlin and Johnson’s son, U.S. Attorney Brendan Johnson. National Democrats view Herseth Sandlin as the stronger of the two.

Tim Johnson, 66, is retiring after three terms in the Senate. There have been questions about Johnson’s political future since December 2006, when he suffered a brain hemorrhage and spent nine months in recovery before he returned to work.

“You have supported me in multiple elections … and more importantly, your patience and prayers allowed me to recover from a life-threatening brain injury,” Johnson said Tuesday.

Despite his absence and the uncertainty surrounding his health, Johnson won re-election in 2008 by 25 points. It was a resounding victory compared with his first re-election: In 2002, Johnson survived a challenge from then-Rep. John Thune by 524 votes — the closest race in the country that cycle.

Two years later, Thune joined Johnson in the Senate.

Johnson, a fourth-generation South Dakotan, started the 2014 cycle in a strong financial position with more than $1.2 million in cash on hand.

Currently chairman of the Banking, Housing, and Urban Affairs Committee, Johnson has been in elected office since 1978. He served in the House for 10 years, as well as both chambers of the state Legislature for a total of eight years.

The Democratic Senatorial Campaign Committee has encouraged senators considering retirement to announce their plans early. Three months into the cycle, Johnson joins four Democratic senators and two Republicans who have already announced their retirements.

“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,” DSCC Chairman Michael Bennet of Colorado said in a statement. “The DSCC will devote all of the resources necessary to win this seat next year, and I am confident that we can elect a new Democratic senator to continue Senator Johnson’s tradition of service to South Dakotans.”

National Republican Senatorial Committee Chairman Jerry Moran of Kansas said the race is a “prime pickup opportunity for Republicans regardless of whose name ends up on the ballot after what’s shaping up to be a contentious Democratic primary.”