Thad Cochran

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FILE - In this Tuesday, Nov. 4, 2014, file photo, Sen. Thad Cochran, R-Miss., speaks to supporters following his victory over Democrat Travis Childers and Reform Party candidate Shawn O'Hara, at his victory party in Jackson, Miss. In a statement released Monday, May 25, 2015, Cochran's office said he married longtime aide Kay Webber in a private ceremony on Saturday in Gulfport, Miss. (AP Photo/Rogelio V. Solis, File)

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Thad Cochran (Mississippi)

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Sen. Thad Cochran, R-Miss., speaks to supporters following his victory over Democrat Travis Childers and Reform Party candidate Shawn O'Hara, at his victory party in Jackson, Miss., in this Nov. 4, 2014, file photo. In a statement released Monday, May 25, 2015, Cochran's office said he married longtime aide Kay Webber in a private ceremony on Saturday in Gulfport, Miss. (AP Photo/Rogelio V. Solis, File)

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** FILE ** State Sen. Chris McDaniel has yet to concede victory to Sen. Thad Cochran. His campaign says it wants to investigate "irregularities" in the primary race as it considers whether to pursue a legal challenge. (Associated Press)

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Sen. Thad Cochran, Mississippi Republican, narrowly defeated state Sen. Chris McDaniel in a runoff for his party's nomination to U.S. Senate. (Associated Press)

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U.S. Sen. Thad Cochran, R-Miss., greets supporters and volunteers at his Canton, Miss., headquarters, Tuesday, June 24, 2014. Cochran is in the Republican primary runoff election against state Sen. Chris McDaniel on Tuesday. (AP Photo/Rogelio V. Solis)

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Challenger: State Sen. Chris McDaniel seeks to unseat six-term Sen. Thad Cochran. (Associated Press)

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Incumbent: Six-term Republican Sen. Thad Cochran was forced into a runoff election for his Senate seat after the recent Mississippi primary failed to result in a clear winner to become the GOP's nominee in November. (Associated Press)