Report: South Dakota's Tim Johnson to leave U.S. Senate in 2014 Sen. Tim Johnson, D-SD (UPI Photo/Roger L. Wollenberg) WASHINGTON, March 25 (UPI) -- Sen. Tim Johnson, D-S.D., will leave the U.S. Senate after finishing his third term in 2014, the Hill reported Monday. Johnson is expected to announce his decision not to seek re-election Tuesday at the University of South Dakota, the Washington publication said. Johnson suffered a stroke in 2006. His retirement would give Republicans a key opportunity to pick up a seat as they work to regain control of the Senate, The Hill said. GOP presidential candidate Mitt Romney carried South Dakota with 57 percent of the vote in the 2012 election.