Update: Updated on March 6, 2018 because of move into new year (Cochran is retiring, which is the 25th best retirement for Dems, because it gives them an extra shot at that seat, but they may have been better off without it. Winning it will be tough and Cochran was likely to miss many votes over the next 9 months, meaning Dems would only have to flip one Republican).
D Governor replaces R Senator, election in 2018
1. Gardner (R-CO)
2. Toomey (R-PA)
3. Daines (R-MT)
D Governor replaces R Senator, special in 2018
4. Kennedy (R-LA)
5. Cassidy (R-LA)
D Governor replaces R Senator with another R (by law), special in 2018
6. Tillis (R-NC)
7. Burr (R-NC)
No replacement of R Senator, special in 2018
8. Hoeven (R-ND)
9. Lankford (R-OK)
10. Inhofe (R-OK)
I Governor replaces R Senator, special in 2018
11. Sullivan (R-AK)
12. Murkowski (R-AK)
R Governor replaces R Senator, special in 2018
13. Collins (R-ME)
14. Johnson (R-WI)
15. Rubio (R-FL)
16. McCain (R-AZ)
17. Isakson (R-GA)
18. Perdue (R-GA)
19. Portman (R-OH)
20. Cornyn (R-TX)
21, Ernst (R-IA)
22. Grassley (R-IA)
23. Scott (R-SC)
24. Graham (R-SC)
25. Cochran (R-MS)
26. Wicker (R-MS)
27. Lee (R-UT)
28. Young (R-IN)
29. Moran (R-KS)
30. Roberts (R-KS)
31. Sasse (R-NE)
32. Alexander (R-TN)
33. Cotton (R-AR)
34. Boozeman (R-AR)
35. Shelby (R-AL)
36. Thune (R-SD)
37. Rounds (R-SD)
38. Paul (R-KY)
39. McConnell (R-KY)
40. Crapo (R-ID)
41. Risch (R-ID)
42. Capito (R-WV)
43. Enzi (R-WY)
R Governor replaces R Senator, term ends in 2018
44. Heller (R-NV)
45. Flake (R-AZ)
46. Cruz (R-TX)
47. Hatch (R-UT)
48. Blunt (R-MO)
49. Fischer (R-NE)
50. Corker (R-TN)
51. Barasso (R-WY)
D Governor replaces D Senator, term ends in 2018
53. Feinstein (D-CA)
54. Gillibrand (D-NY)
55. Menendez (D-NJ)
56. Booker (D-NJ)
57. Carper (D-DE)
58. Kaine (D-VA)
59. Klobucher (D-MN)
60. Smith (D-MN)
61. Casey (D-PA)
62. Tester (D-MT)
D Governor replaces D Senator, special in 2018
63. Schatz (D-HI)*
64. Harris (D-CA)
65. Markey (D-MA)
66. Warren (D-MA)
67. Schumer (D-NY)
68. Murray (D-WA)
69. Cantwell (D-WA)
70. Murphy (D-CT),
71. Coons (D-DE)
72. Warner (D-VA)
73. Bennet (D-CO)
No replacement of D Senator, special in 2018
74. Blumenthal (D-CT)
75. Reed (D-RI)
76. Whitehouse (D-RI)
77. Wyden (D-OR)
78. Merkley (D-OR)
79. Heitkamp (D-ND)
R Governor replaces D Senator, special in 2018
80. Leahy (D-VT)
81. Sanders (I-VT)
82. Baldwin (D-WI)
R Governor replaces D Senator
83. Cardin (D-MD)
84. Van Hollen (D-MD)
85. Durbin (D-IL)
86. Duckworth (D-IL)
87. Udall (D-NM)
88. Heinrich (D-NM)
89. King (I-ME)
90. Masto (D-NV)
91. Shaheen (D-NH)
92. Hassan (D-NH)
93. Stabenow (D-MI)
94. Peters (D-MI)
95. Nelson (D-FL)
96. Brown (D-OH)
97. McCaskeill (D-MO)
98. Donnelly (D-IN)
99. Jones (D-AL)
100. Manchin (D-WV)
+ Governor is an Independent, has been a Republican and ran with the Democratic nominee
*Party leaders pick the possible replacements and governor selects from them
++if vacancy occurs before March 2018, otherwise seat empty till Nov 2018
