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Status: Running for reelection In this Aug. 13, 2009, file photo, Sen. Mark Begich, D-Alaska, talks about his trip on Wednesday to Bethel and Hooper Bay in Anchorage, Alaska. (AP Photo/Al Grillo, File)

Status: Running for reelection In this Aug. 2, 2013 file photo. Sen. Mark Pryor, D-Ark. is interviewed at his campaign office in Little Rock, Ark. (AP Photo/Danny Johnston, File)

Status: Running for reelection Sen. Mark Udall (D-CO) speaks at a campaign rally for U.S. President Barack Obama at Sloan's Lake Park on October 4, 2012 in Denver, Colorado. (Photo by Doug Pensinger/Getty Images)

Status: Running for reelection Sen. Chris Coons, D-Del., watches election returns at the Delaware Democratic Party Election Night at the Queen theater in Wilmington, Del., Tuesday, Nov. 6, 2012. (AP Photo/Cliff Owen)

Status: Retiring FILE - In this April 21, 2010 file photo, Sen. Saxby Chambliss, R-Ga. speaks on Capitol Hill in Washington. (AP Photo/Charles Dharapak, File)

Status: Running for term ending 1/3/17 Brian Schatz smiles as he talks with reporters on the tarmac after deplaning Air Force One, Thursday, Dec. 27, 2012, at Andrews Air Force Base, Md. (AP Photo/Carolyn Kaster)

Status: Running for reelection Newly elected Republican Senator Jim Risch, of Idaho poses for a photo in the office of Senate Minority Leader Mitch McConnell November 17, 2008 at the US Capitol, in Washington, D.C. (MANDEL NGAN/AFP/Getty Images)

Status: Running for reelection Senate Defense subcommittee Chairman Sen. Richard Durbin, D-Ill. presides over the subcommittee's hearing to examine Defense Department leadership, Tuesday, June 11, 2013, on Capitol Hill in Washington. (AP Photo/J. Scott Applewhite)

Status: Retiring FILE - In this Dec. 28, 2012 file photo Democratic Iowa Sen. Tom Harkin speaks on Capitol Hill in Washington. (AP Photo/Susan Walsh)

Status: Running for reelection U.S. Sen. Pat Roberts, a Kansas Republican, speaks during a groundbreaking ceremony Tuesday, May 28, 2013, on a utility plant for the new federal biosecurity lab in Manhattan, Kan. (AP Photo/John Milburn)

Status: Running for reelection FILE - In this July 30, 2013 file photo, Senate Minority Leader Mitch McConnell of Ky. speaks with reporters on Capitol Hill in Washington. (AP Photo/J. Scott Applewhite, File)

Status: Running for reelection FILE - In this Dec. 5, 2012 file photo, Sen. Mary Landrieu, D-La. is seen on Capitol Hill in Washington. (AP Photo/Manuel Balce Ceneta, File)