SIOUX FALLS - Democratic U.S. Sen. Tim Johnson of South Dakota says he's retiring at the end of his term, acknowledging he remains limited by a recent "health crisis."

The 66-year-old said at a news conference that "my right arm and right leg aren't what they used to be and my speech is not entirely there."

Appearing in his hometown Tuesday, Johnson made it official he will not seek a fourth term. Lingering effects from a 2006 brain hemorrhage have slowed his speech and caused him to use a motorized scooter.

Johnson says it'll be "strange" to not have an election looming. Johnson said, "I've run for elections 36 years in a row, and it's now time to give it up."

He added with a laugh, "I'm certain I can get over it."

Johnson also joked that his wife wanted him to run again but that he refused. He said it's time to move on to spend more time with his six grandchildren "in the state we love."

The move gives Republicans one of their best chances to gain a seat and regain Senate control.

The only announced Republican candidate so far in the GOP-trending state is Mike Rounds, who was governor for two terms after being elected in 2002.

Rounds on Tuesday thanked retiring Democratic Sen. Tim Johnson for his many years of public service to South Dakota.

Rounds, a Republican, is running for Johnson's seat in 2014. Rounds in a statement says that although he and Johnson disagreed on policy over the years, he has always respected Johnson for his hard work on behalf of the state. Rounds wished Johnson and his wife, Barb, the very best.

A former U.S. senator whose 22-year congressional career ended in 1996 when he was defeated by Johnson says Johnson's decision to continue to serve despite medical difficulties made him a figure of national courage.

Republican Larry Pressler says Johnson served the state with great courage. Pressler says Johnson's work on the Appropriations Committee made a huge difference for South Dakota on water and education projects and Native American issues.

Despite the party difference, Pressler endorsed Johnson in 2008 in his bid to hold his seat against challenger Joel Dykstra. Pressler says he made the endorsement based on his childhood stuttering memories of how hard it is when you have something to say but can't easily speak out.

President Obama on Tuesday commended Johnson for his work to improve the lives of South Dakotans. Obama says that in Johnson's more than three decades in office, he worked tirelessly to protect the environment, help rural and Native American communities and build a better financial system.

Obama says Johnson's return to the Senate floor following his life-threatening brain injury in 2006 was inspiring.

The president says he wishes Johnson, his wife, Barb, and their family the best.

Meanwhile, Former U.S. Rep. Stephanie Herseth Sandlin has been named as a possible successor to Johnson, but she is declining to say whether she will run for the post.

Herseth Sandlin told The Associated Press on Tuesday that she appreciates the encouragement she's received but added, "I haven't focused on the future political opportunities."

Another possible successor discussed by South Dakota Democrats is Johnson's son, Brendan Johnson, the state's U.S. attorney. The senator says he hasn't "discussed in detail" who might follow him.

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