John Thune

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Senate Majority Leader Sen. Mitch McConnell of Ky., accompanied by, from left, Sen. John Barrasso, R-Wyo., Sen. John Thune, R-S.D., and Senate Majority Whip John Cornyn of Texas, speaks during a news conference on Capitol Hill in Washington, Tuesday, May 5, 2015, after a policy luncheon. (AP Photo/Evan Vucci)

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FILE - In this Jan. 16, 2015 file photo. South Dakota Republican Sen. John Thune speaks at the Performing Arts Center in Rapid City, S.D. First-quarter federal fundraising numbers released Wednesday, April 15, 2015, show Thune brought in roughly $793,000 and that he is sitting on $10.37 million. Thune is up for re-election in 2016. (Chris Huber/Rapid City Journal via AP, File)

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Senate Majority Leader Mitch McConnell of Kentucky accompanied by Sen. John Thune, South Dakota Republican, right, meets with reporters on Capitol Hill in Washington, Tuesday, Feb. 24, 2015, to talk about his move to disentangle one of two contested immigration measures from the Homeland Security budget and debate the issues separately, as the Senate faces an impasse over provisions attached to the spending bill aimed at blocking President Barack Obama's executive actions on immigration. (AP Photo/J. Scott Applewhite)

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Sen. John Thune of South Dakota said it would be politically treacherous for Republicans to refuse to pass the homeland security bill, especially amid a spate of attacks and plots by Islamist terrorists. (Associated Press)

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Sen. John Thune, of South Dakota, speaks at the South Dakota Republican Convention in Rapid City, S.D., in this June 20, 2014, file photo. (AP Photo/Toby Brusseau, File)

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Delmar Strunk laughs after greeting Senator John Thune Wednesday, May 28, 2014 before a ceremony at city hall in Scotland, S.D. to award him a Bronze Star for his involvement as one of Merrill’s Marauders in WWII. Strunk served in the war from October 1943 to April 1945. His service included a four-month mission behind enemy lines in the Burmese jungle to attack Japanese bases and supply routes. Only about 200 of the 3,000 members of the unit known as Merrill's Marauders survived. After the war, Strunk's personnel records were lost, and he never received the medals that he was due. Military historian and author Jon Hittle later discovered Strunk's involvement in the war and enlisted U.S. Sen. John Thune, R-S.D., to help secure the medals for Strunk. (AP Photo/Argus Leader, Elisha Page)

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Senate Minority Leader Mitch McConnell, R-Ky., joined by Sen. John Barrasso, R-Wyo., left, and Sen. John Thune, R-S.D., rear left, speaks to reporters after a Republican caucus meeting, at the Capitol in Washington, Tuesday, May 6, 2014. (AP Photo/J. Scott Applewhite)

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Senate Minority Leader Mitch McConnell of Ky., center, and GOP lawmakers, from left, Sen. John Barrasso, R-Wyo., Sen. John Thune, R-S.D., and Senate Minority Whip John Cornyn of Texas, talk to reporters after a GOP caucus meeting, Tuesday, April 29, 2014, on Capitol Hill in Washington. (AP Photo)