I agree with your point about "something extremely damaging." The Mueller investigation's final report will need to detail serious crimes beyond obstruction of justice.

I count about 10 republican senators who have been very openly anti-Trump. Those should be relatively easy to convince to cross over. If you look at senators who have severely chastised Trump's behavior on multiple occasions, that list grows even larger. There are a lot of republican senators who seem to strongly dislike Trump but won't say anything for fear of angering his base. I think there are enough of those that might be willing to cross the aisle when the investigation is complete.