1. Nevada — GOP Sen. Dean Heller running for reelection
2. Missouri — Democratic Sen. Claire McCaskill running for reelection
3. Indiana — Democratic Sen. Joe Donnelly running for reelection
4. Arizona — Open; GOP Sen. Jeff Flake retiring
5. North Dakota — Democratic Sen. Heidi Heitkamp running for reelection
6. West Virginia — Democratic Sen. Joe Manchin running for reelection
7. Ohio — Democratic Sen. Sherrod Brown running for reelection
8. Florida — Democratic Sen. Bill Nelson running for reelection
9. Montana — Democratic Sen. Jon Tester running for reelection
10. Wisconsin — Democratic Sen. Tammy Baldwin running for reelection
Honorable mentions (in alphabetical order): Michigan, Minnesota, New Jersey, Pennsylvania, Tennessee and Texas