In reply to elsewhere:
The way I see it, he must have two theoretical endgames on his mind.
1. If I don't do it, we all go to prison for a long time, it's worth a shot despite legally admitting some guilt.
2. I don't think I will be convicted by the senate, and after my aquittal I will fight and dodge any legal battles to keep myself out of prison, therefore I should be able to avoid the humiliation of needing to pardon myself.
I think 1 is more likely for a desparate despot, but 2 makes some sense too.