Sen. Tim Johnson, D-S.D., will announce Tuesday that he will not seek re-election next year, several media outlets report, becoming the seventh senator to retire and giving Republicans one of their best chances to gain a seat.

Johnson, who battled back after suffering a life-threatening brain hemorrhage in 2006, is chairman of the Banking, Housing and Urban Affairs Committee.

The senator has scheduled an announcement at 3 p.m. Tuesday at the University of South Dakota in Vermillion, his alma mater, to disclose his plans, ABC reported. One Democratic source in South Dakota and another official in Washington, both speaking off the record to avoid upstaging the official announcement, confirmed the decision.

Johnson, 66, has spent the past 28 years in Washington. He served three terms in the Senate and five terms in the House. He is one of the last Democratic senators to represent the Plains, where Republicans have steadily been picking up seats.

While giving a radio interview on Dec. 13, 2006, Johnson suffered bleeding in the brain. He returned to the Senate the next year to a standing ovation from Republicans and Democrats alike, but his speech has never fully recovered from the stroke and he uses a motorized scooter. Nevertheless, he was re-elected in 2008.

This will be South Dakota’s first open Senate race in 35 years. Candidates were already eyeing the seat, including former Gov. Mike Rounds and Rep. Kristi Noem, both Republicans. Rounds announced his intention to run in November, shortly after President Barack Obama lost South Dakota by 18 percentage points.

On the Democratic side, the senator's son, Brendan Johnson, the U.S. attorney in South Dakota, is a possible contender, as well as former Rep. Stephanie Herseth Sandlin.

In addition to Johnson, the Democratic senators not seeking re-election next year are John D. (Jay) Rockefeller IV of West Virginia, Tom Harkin of Iowa, Frank Lautenberg of New Jersey and Carl Levin of Michigan. The two retiring Republican senators are Saxby Chambliss of Georgia and Mike Johanns of Nebraska.

Democrats currently have a 55-45 voting edge in the Senate. They have 21 Senate seats to defend in 2014, including in seven states that voted for GOP presidential nominee Mitt Romney last November. Republicans will defend 14 seats.