Updated

Democratic U.S. Sen. Tim Johnson of South Dakota announced Tuesday that he would retire at the end of his term, acknowledging he remains limited by a recent "health crisis" and wants to spend more time with his grandchildren.

Appearing Tuesday in his hometown of Vermillion, 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, which he used to get to the news conference Tuesday.

"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, 66, said from the University of South Dakota in his hometown of Vermillion.

Citing his age and a desire to spend more time with his family, Johnson said "it is time for me to say goodbye." He also ruled out running for any other office when his term ends in 2014.

"I look forward to serving the remaining two years as the country is facing difficult times on many fronts and I will work every day to find a bipartisan solution to these challenges," he said.

Former Gov. Mike Rounds, a Republican, had announced plans last year to challenge Johnson, the chairman of the Senate Banking Committee, and figures to give Republicans one of their best opportunities at picking up a seat.

Johnson joins Democratic Sens. Carl Levin of Michigan, Tom Harkin of Iowa, Jay Rockefeller of West Virginia, and Frank Lautenberg of New Jersey as seasoned and influential Democrats departing the chamber, where Republicans need to gain six seats to take control. Among those states, West Virginia was carried by Republican presidential candidate Mitt Romney last year.

Two Republican senators have announced their retirements, both in Republican-performing states Georgia and Nebraska.

By stepping aside, Johnson, who had never lost an election, leaves South Dakota Democrats searching their youthful prospects for a potential successor.

"I believe South Dakota moves into the top slot as the most likely Republican pickup," said Greg Strimple, a Republican pollster and past consultant to the National Republican Senatorial Committee.

Aware that Johnson might decide to retire, Democrats in South Dakota and nationally have discussed possible successors on the ticket, chiefly Johnson's son Brendan Johnson and former U.S. Rep. Stephanie Herseth Sandlin.

Reached by phone Tuesday, Herseth Sandlin declined to say whether she would run.

"While I appreciate the encouragement I've received I haven't focused on the future political opportunities," she told The Associated Press.

Brendan Johnson, 37, who was appointed U.S. attorney for South Dakota in 2009, has never held elected office and faced questions about his father's involvement in the confirmation process.

The senator declined to comment on whether his son might seek the job. He said he had told him of his decision but didn't say whether he would urge him to run.

Like the younger Johnson, Herseth Sandlin is a well-known name in the state and has a fundraising base. But the 42-year-old could face problems in a Democratic primary in light of her opposition to the 2010 Affordable Care Act, a position that is out of step with a majority of party loyalists.