Former Republican U.S. Sen. Larry Pressler, an independent candidate for U.S. Senate in South Dakota, speaks at a news conference, Tuesday, July 8, 2014, in Sioux Falls, S.D. (AP Photo/Dirk Lammers)
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)
U.S. Senate candidate Bruce Braley speaks Sunday, Oct. 27, 2013, during the Bruce Blues & BBQ fundraiser for the Braley senate campaign at the Iowa State Fairgrounds in Des Moines, Iowa. (AP Photo/Scott Morgan)
Shenna Bellows candidate for U.S. Senate speaks at the Maine Democratic Convention in Bangor, Maine, Saturday, May 31, 2014. (AP Photo/Michael C. York)
Shenna Bellows candidate for U.S. Senate speaks at the Maine Democratic Convention in Bangor, Maine, Saturday, May 31, 2014. (AP Photo/Michael C. York)
Shenna Bellows candidate for U.S. Senate speaks at the Maine Democratic Convention in Bangor, Maine, Saturday, May 31, 2014. (AP Photo/Michael C. York)
In this May 31, 2014 photo, Republican U.S. Senate candidate Mike McFadden cheers at the Minnesota Republican Party Convention at the Rochester Civic Center in Rochester, Minn. Minnesota Republicans endorsed McFadden on Saturday for U.S. Senate. (AP Photo/The Star Tribune, David Joles) MANDATORY CREDIT; ST. PAUL PIONEER PRESS OUT; MAGS OUT; TWIN CITIES TV OUT
In this May 31, 2014 photo, Republican U.S. Senate candidate Mike McFadden addresses delegates at the Minnesota Republican Party Convention at the Rochester Civic Center in Rochester, Minn. Minnesota Republicans endorsed McFadden on Saturday for U.S. Senate. (AP Photo/The Star Tribune, David Joles) MANDATORY CREDIT; ST. PAUL PIONEER PRESS OUT; MAGS OUT; TWIN CITIES TV OUT