- West Virginia - Incumbent: Democrat Joe Manchin
- Montana - Incumbent: Democrat Jon Tester
- Ohio - Incumbent: Democrat Sherrod Brown
- Arizona - Incumbent: Independent Kyrsten Sinema
- Nevada - Incumbent: Democrat Jacky Rosen
- Wisconsin - Incumbent: Democrat Tammy Baldwin
- Pennsylvania - Incumbent: Democrat Bob Casey
- Michigan - Incumbent: Democrat Debbie Stabenow (retiring)
- Texas - Incumbent: Republican Ted Cruz
- Florida - Incumbent: Republican Rick Scott
Also unlikely that Arizona will replace Sinema with a Republican.
Still, Democrats certainly have their work cut out for them to retain control of the Senate this cycle. Will probably be a lot easier to flip the House and retain the Presidency.
Doubt that Manchin and Tester will be reelected. That in itself would flip the Senate if everything else stays the same.