Someone asked what senators are up for reelection in 2020 and I can’t find the post now so I’m piggybacking this to your post; Democrats (12) and Republicans (32) up 2020....
Democrats up for reelection in 2020:
Alabama: Doug Jones (Running) Current age: 64
Delaware: Chris Coons (Running) Current age: 54
Illinois: Dick Durbin (Intent unknown) Current age: 73
Massachusetts: Ed Markey (Running) Current age: 71
Michigan: Gary Peters (Intent unknown) Current age: 59
Minnesota: Tina Smith (Running) Current age: 60
New Hampshire: Jeanne Shaheen (Intent unknown) Current age: 71
New Jersey: Cory Booker (Intent Unknown) Current age: 48
New Mexico: Tom Udall (Running) Current age: 69
Oregon: Jeff Merkley (Intent unknown) Current age: 61
Rhode Island: Jack Reed (Intent unknown) Current age: 68
Virginia: Mark Warner (Intent unknown) Current age: 63
Republicans up for reelection in 2020:
Alaska: Dan Sullivan (Intent unknown) Current age: 53
Arizona: Martha McSally (Running) Current age: 52
Arkansas: Tom Cotton (Running) Current age: 40
Colorado: Cory Gardner (Running) Current age: 43
Georgia: David Perdue (Running) Current age: 68
Idaho: John Risch (Intent unknown) Current age: 74
Iowa: Joni Ernst (Running) Current age: 47
Kansas: Pat Roberts (Retiring) Current age: 81
Kentucky: Mitch McConnell (Running) Current age: 75
Louisiana: Bill Cassidy (Intent unknown) Current age: 60
Maine: Susan Collins (Running) Current age: 66
Mississippi: Cindy Hyde-Smith (Intent unknown) Current age: 59
Montana: Steve Daines (Intent unknown) Current age: 55
Nebraska: Ben Sasse (Intent unknown) Current age: 45
North Carolina: Thom Tillis (Running) Current age: 57
Oklahoma: Jim Inhofe (Intent unknown) Current age: 83
South Carolina: Lindsey Graham (Running) Current age: 63
South Dakota: Mike Rounds (Intent unknown) Current age: 63
Tennessee: Lamar Alexander (Retiring) Current age: 77
Texas: John Cornyn (Running) Current age: 65
West Virginia: Shelley Moore Capito (Intent unknown) Current age: 64
Wyoming: Mike Enzi (Intent unknown) Current age: 73