These Republican seats are up for re-election in 2020.
"Don't agonize, organize!"

Alaska:

Date of Primary: 08/18/2020

Republican Primary candidate: Dan Sullivan
Democratic primary candidates: No Democratic candidate has filed to run in this election.

Arizona:

Date of Primary: 08/04/2020

Republican Primary candidate: Martha McSally

Democratic primary candidates:

• Mohammad Arif
• Sheila Bilyeu
• Mark Kelly
• Juan Angel Vasquez

Arkansas:

Date of Primary: 03/03/2020

Republican Primary candidate: Tom Cotton

Democratic primary candidates:
The Democratic primary was canceled.

Colorado:

Date of Primary:  06/30/2020

Republican Primary candidate: Cory Gardner

Democratic primary candidates:

• Diana Bray
• Lorena Garcia
• David Goldfischer
• John Hickenlooper
• Dustin John Leitzel
• Christopher Hawkins Critter Milton
• Andrew Romanoff
• Stephany Rose Spaulding
• Erik Underwood
• Michelle Ferrigno Warren
• Trish Zornio

Georgia:

Date of Primary: 05/19/2020

Republican Primary candidate:

• David Perdue
• Kelley Loeffler (appointed to the seat vacated by Isakson)

Democratic primary candidates:

• Sarah Riggs Amico
• Akhenaten Amun
• Marckeith DeJesus
• Maya Dillard Smith
• James Knox
• Jon Ossoff
• Harold Shouse
• Teresa Tomlinson
• Elaine Whigham Williams

Idaho:

Date of Primary: 05/19/2020

Republican Primary candidate: James E Risch

Democratic primary candidates:

• Nancy Harris
• Travis Oler
• James Vandermaas

Iowa:

Date of Primary: 06/02/2020

Republican Primary candidate: Joni Ernst

Democratic primary candidates:

• Michael Franken
• Kimberly Graham
• Theresa Greenfield
• Eddie Mauro
• Cal Woods

Kansas:

Date of Primary: 08/04/2020

Republican Primary candidate: Pat Roberts

Democratic primary candidates:

• Barbara Bollier
• Usha Reddi
• Adam Smith
• Robert Tillman

Kentucky:

Date of Primary: 05/19/2020

Republican Primary candidate: Mitch McConnell

Democratic primary candidates:

• Jimmy Ausbrooks
• Charles Booker
• Mike Broihier
• Maggie Jo Hilliard
• Andrew Maynard
• Amy McGrath
• Eric Rothmuller
• John Sharpensteen
• Bennie Smith
• Mary Ann Tobin

Louisiana:

Date of Primary: ?

Republican Primary candidate: John Kennedy

Democratic primary candidates:

• Antoine Pierce 

Maine:

Date of Primary: 06/09/2020

Republican Primary candidate: Susan M Collins

Democratic primary candidates:

• Michael Bunker
• Sara Gideon
• Bre Kidman
• Ross LaJeunesse
• Betsy Sweet

Mississippi:

Date of Primary: 03/10/2020

Republican Primary candidate: Cindy Hyde-Smith

Democratic primary candidates:

• Tobey Bartee
• Jensen Bohren
• Mike Espy

Montana:

Date of Primary: 06/02/2020

Republican Primary candidate: Steve Daines

Democratic primary candidates:

• Wilmot Collins
• Mike Knoles
• John Mues
• Cora Neumann

Nebraska:

Date of Primary: 05/12/2020

Republican Primary candidate: Ben Sasse

Democratic primary candidates:

• Chris Janicek
• Dennis Macek
• Angie Philips
• Alisha Shelton

North Carolina:

Date of Primary: 03/03/2020

Republican Primary candidate: Thom Tillis

Democratic primary candidates:

• Cal Cunningham
• Trevor Fuller
• Atul Goel
• Erica Smith
• Steve Swenson

Oklahoma:

Date of Primary: 06/30/2020

Republican Primary candidate: James M Inhofe

Democratic primary candidates:

• Dylan Billings
• Elysabeth Britt
• Abby Broyles
• Tyler Dougherty
• Bevon Rogers
• Paul Tay
• Mike Workman

South Carolina:

Date of Primary: 06/09/2020

Republican Primary candidate: Lindsey Graham

Democratic primary candidates:

• Jaime Harrison
• Matthew Knights
• William Stone
• Justin Wooton

South Dakota:

Date of Primary: 06/02/2020

Republican Primary candidate: Mike Rounds

Democratic primary candidates:

• Dan Ahlers
• Clara Hart

Tennessee:

Date of Primary: 08/06/2020

Republican primary candidates: Lamar Alexander (retiring)

Democratic primary candidates:

• Marquita Bradshaw
• Tharon Chandler
• Gary Davis
• James Mackler
• Diana Onyejiaka

Texas:

Date of Primary: 03/03/2020

Republican primary candidates: John Cornyn

Democratic primary candidates:

• Chris Bell
• Michael Cooper
• Amanda Edwards
• Jack Daniel Foster Jr.
• Annie Garcia
• Victor Harris
• Mary Jennings Hegar
• Sema Hernandez
• D.R. Hunter
• Adrian Ocegueda
• Cristina Tzintzún Ramirez
• Royce West

West Virginia:

Date of Primary: 05/12/2020

Republican primary candidates: Shelley Moore Caputo

Democratic primary candidates:

• Richard Ojeda
• Richie Robb
• Paula Jean Swearengin

Wyoming:

Date of Primary: 08/18/2020

Republican primary candidates: Michael B Enzi

Democratic primary candidates:

• Merav Ben-David
• Chuck Jagoda
• Yana Ludwig

Data compiled with information from this website and Ballotpedia