As expected, Sen. Tim Johnson (D-S.D.) announced Tuesday afternoon that he will retire in 2014, giving South Dakota an open Senate seat for the first time since 1978. Republicans are already starting to split on a candidate for the race, which is heavily tilted in the GOP's favor.

“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 news conference at his alma mater, the University of South Dakota. "As much as [my wife] Barbara would like me to run again, I have to say no," he joked.

Johnson, the chairman of the powerful Senate Banking, Housing and Urban Affairs Committee, started his political career as a state legislator in 1979 and has held public office ever since. The senator said Tuesday that it would be "strange" not to have an election looming, "but I'm certain that I can get over it."

Former South Dakota governor Mike Rounds (R) announced his candidacy for the Senate seat long before Johnson retired. But the Senate Conservatives Fund, a conservative outside group founded by former South Carolina senator Jim DeMint, has come out against a run by Rounds.

"This race is too important to concede it to another moderate politician who won't fight for limited government," SCF Executive Director Matt Hoskins said in a statement Tuesday. Rep. Kristi Noem (R) might challenge Rounds in the Republican primary.

Whoever Republicans pick has the edge here -- Mitt Romney won the state by 18 points last fall. Asked if he could have held his seat had he run again, Johnson replied,  "I've never been beaten."

On the Democratic side, all eyes are on two potential candidates: Johnson's son, U.S. Attorney Brendan Johnson, and former Rep. Stephanie Herseth Sandlin, who was defeated by Noem in 2010. The elder Johnson said that there were "several good candidates out there" and that he would "let the process play out" on its own.

Democratic Senatorial Campaign Committee Chairman Michael Bennet (D-Colo.) pledged in a statement "to devote all of the resources necessary to win this seat." National Republican Senatorial Committee Chairman Jerry Moran (R-Kan.) called it "a prime pick up opportunity for Republicans regardless of whose name ends up on the ballot" on the Democratic side.