SIOUX FALLS, South Dakota–Sen. John Thune (R-S.D.) won the Republican primary on his way to an all-but-assured fourth term in the U.S. Senate on June 7, beating out two ultra-conservative candidates. Thune, U.S. Senate Minority Whip, edged out two conservative Republican candidates, Bruce Whalen and Mark Mowry. “I want to thank Republicans in South Dakota for…