Murkowski and/or Collins could switch to Indie and Grassley and/or McCain could die/retire for health reasons. There could also be some appointments to the cabinet - I do not expect there to be a lot of stability.
But, yeah, I woud say 80% sounds right.
Another interesting possibility is an unfilled vacancy in vice-presidency. Suppose that, for whatever reason, with the Senate at 50/50 Pence gets promoted. Of course, a new VP would get to break the Senate tie - but the new VP would have to be confirmed by the Senate
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