I read that you need a +8 rating before the AI even has a hope to favour you, but I found this to not be true. For example, my last game where I held a diplo vote, I has pissed off Isabella the entire game. Kept on declaring war on her refriends, making refusals, and kept on demanding techs. Even though we shared the same religion, my overall rating was NEGATIVE 10.
ghandi built the UN, and because I had a LOT of people I got on the list. I was very shocked that Isabela voted for me!!! Everyone always votes for Ghandi! I suppose because he had a differenent religion, and Isabella being a religious nutcase, she still voted for me (her bitter enemy) over her friend Ghandi. That's a first. So it seems, at least for some Ai personalities, the -/+ modifiers themselves are meaningless. There are other items at work here under the hood.
I've also noticed time and time again, that the AI doesn't mind voting for you when it knows its votes wont count. I.E. not enough to give you a diplo win. But once you get your population big enough to cross the payline, they suddenly shut down on you and either abstain or vote for someone else.