But it is only 2/3 of those voting.
So as with the vote to end slavery in England... Send one side off to the race tracks for the day, and the vote is done before they get back.
So attendance, attendance, attendance !
Two points ... and not that this will never happen but there is no such thing as a sneaky vote in the senate. All this stuff is agreed. They are all treated like monarchs.
The other point is that when sitting in impeachment trial, it is even harder to hide the fact that just a few senators are in the room.