Here are the senators up for reelection this November.
SENATORS UP FOR RE-ELECTION IN 2016
DEMOCRATS
Michael Bennet (Colorado)
Richard Blumenthal (Connecticut)
Barbara Boxer (California) retiring in 2016
Patrick Leahy (Vermont)
Barbara Mikulski (Maryland) retiring in 2016
Patty Murray (Washington)
Harry Reid (Nevada) retiring in 2016
Brian Schatz (Hawaii)
Charles Schumer (New York)
Ron Wyden (Oregon)
REPUBLICANS
Kelly Ayotte (New Hampshire)
Roy Blunt (Missouri)
John Boozman (Arkansas)
Richard Burr (North Carolina)
Dan Coats (Indiana) retiring in 2016
Mike Crapo (Idaho)
Chuck Grassley (Iowa)
John Hoeven (North Dakota)
Johnny Isakson (Georgia)
Ron Johnson (Wisconsin)
Mark Kirk (Illinois)
James Lankford (Oklahoma)
Mike Lee (Utah)
John McCain (Arizona)
Jerry Moran (Kansas)
Lisa Murkowski (Alaska)
Rand Paul (Kentucky)
Rob Portman (Ohio)
Marco Rubio (Florida) retiring in 2016
Tim Scott (South Carolina)
Richard Shelby (Alabama)
John Thune (South Dakota)
Pat Toomey (Pennsylvania)
David Vitter (Louisiana)
Rubio said he will not run. Has he changed his mind on that?