Tim Johnson

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FILE - In this April 12, 2014 file photo, the five candidates seeking the GOP nomination to run for the U.S. Senate seat being vacated by the retiring Democratic Sen. Tim Johnson, from left, former South Dakota Gov. Mike Rounds, state Rep. Stace Nelson, Sioux Falls physician Annette Bosworth, Yankton attorney Jason Ravnsborg and state Sen. Larry Rhoden, participate in a debate in Pierre, S.D. The five will face off again in a live television debate Thursday, May 15, 2014, in Vermillion, S.D. (AP Photo/Capital Journal, Nick Lowrey, File)

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This Feb. 17, 2014, photo shows Sioux Falls physician Annette Bosworth talking about her candidacy for the U.S. Senate seat being vacated by Sen. Tim Johnson in Sioux Falls, S.D. Bosworth will face four opponents in the Republican primary on June 3. (AP Photo/Dirk Lammers)

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ADVANCE FOR USE MONDAY, APRIL 7 AND THEREAFTER - In this Feb. 20, 2014 photo, former South Dakota Gov. Mike Rounds talks about his candidacy for the U.S. Senate seat being vacated by Sen. Tim Johnson in Sioux Falls, S.D. Rounds will face four opponents in the Republican primary on June 3. (AP Photo/DIrk Lammers)

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ADVANCE FOR USE MONDAY, APRIL 7 AND THEREAFTER - In this Feb. 20, 2014 photo, former South Dakota Gov. Mike Rounds talks about his candidacy for the U.S. Senate seat being vacated by Sen. Tim Johnson in Sioux Falls, S.D. Rounds will face four opponents in the Republican primary on June 3. (AP Photo/DIrk Lammers)