It isn't even clear what exactly will be voted on. Will it be the most recent Senate draft? Will it be another draft? Will it just be a clean ACA repeal? Will it need 60 or 51 votes? On that last bit, the Senate Parliamentarian ruled multiple key aspects of the latest Trumpcare draft to require a 60 vote super majority rather than being able to pass through reconciliation rules. (Not impossible to get around this in the same manner that the Senate GOP confirmed Gorsuch earlier this year.)
Key Senators to watch come into a couple groups:
1. Senators that may vote against the latest draft but could vote for a clean repeal:
Also worth noting that McCain is likley to miss this next week as well. Therefore, Senate Republicans can only afford to lose 1 vote and not 2 before the bill fails. Therefore, McConnell, for just a clean repeal, will need to flip 3 of the second group and for the latest draft would need to flip both of the first group and three of the second group.
I included the Trump Tweet in this post to highlight the elephant in the room: Trump could turn his fury at any Senator that opposes him on this. Most vulnerable Republicans are Flake and Heller. Trump reportedly met with Flake's 2018 primary opponent this last week. Heller has already drawn a credible Democratic challenger and Flake is expected to draw one soon as well in the 47 year old Mayor of Phoenix, Greg Stanton. There are also rumors that Mike Kelly (former astronaut and husband of former Congresswoman Gabrielle Giffords) is considering a run.