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By Jeff Zeleny

Sen. Tim Johnson, D-S.D., is set to announce Tuesday that he will not seek re-election, according to two sources, becoming the seventh incumbent to retire after next year.

Johnson, who battled back after suffering a life-threatening brain hemorrhage in 2006, is chairman of the Banking, Housing and Urban Affairs Committee. But in political terms he is something else: A Democrat serving in a deeply conservative state. His retirement gives Republicans another open seat to pursue in their quest to win control of the Senate.

He has scheduled an announcement at 3 p.m. Tuesday at the University of South Dakota in Vermillion, his alma mater, to disclose his plans. One Democratic source in South Dakota and another official in Washington, both of whom spoke on condition of anonymity 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 has faced some of the Senate's most competitive fights for re-election and is one of the last Democratic senators to represent the Midwestern Plains, where Republicans have steadily been picking up seats.

In a radio interview Dec. 13, 2006, speaking to a station back home in Yankton, S.D., 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.

The retirement creates the first open Senate seat in South Dakota in 35 years.

Even before his decision to retire, several prospective candidates were already eyeing the seat, including former Gov. Mike Rounds and Representative Kristi Noem, both Republicans. 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 Representative Stephanie Herseth Sandlin.

The face of the Senate is swiftly changing. In addition to Johnson, the Democratic senators not seeking re-election next year are John Rockefeller of West Virginia, Tom Harkin of Iowa, Frank Lautenberg of New Jersey and Carl Levin of Michigan. The two retiring Republicans senators are Saxby Chambliss of Georgia and Mike Johanns of Nebraska.

As Republicans rebuild from losing the White House race and seats in the House and Senate last year, party leaders and strategists are intently focused on trying to win control of the Senate in 2014. Republicans must pick up six seats to wrestle the majority from Democrats.

ABC News' Sunlen Miller contributed to this report.