How is the “abyss” calculated?
Abyss is Total Approve minus Strong Disapprove.
The premise is Total Approve represents the most votes a President can get, and the Strong Disapprove represents those who will come out to specifically vote against the President. A negative Abyss rating generally is seen as certain defeat, because you can't overcome the Strong Disapprovers even if you manage to motivate even your marginal supports to go to the polls.