United States Senate

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Former Massachusetts U.S. Senator Scott Brown announces his plans to run for the U.S. Senate in New Hampshire Thursday, April 10, 2014 in Portsmouth, N.H. Brown hopes to unseat U.S. Sen. Jeanne Shaheen, D-N.H. (AP Photo/Jim Cole)

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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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State Rep. Stace Nelson, left, a Republican candidate for U.S. Senate, and Democratic candidate Rick Weiland speak at a joint news conference, Monday, April 7, 2014, in Sioux Falls, S.D. The candidates criticized former Gov. Mike Rounds, the Republican frontrunner in the race, for his millions of dollars raised from out-of-state sources. (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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In this Thursday, March 6, 2014 photo, North Carolina House of Representatives Speaker Thom Tillis, R-Mecklenburg, addresses attendees during Wilson County's Republican Convention, held on the campus of Wilson Community College in Wilson, N.C. As one of the architects of the Republican revolution arriving in Raleigh three years ago, Tillis has received both praise and consternation among Republicans during his U.S. Senate campaign. (AP Photo/The Wilson Times, Brad Coville)

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