Probably the major reason for the poll changing is the definition and how they asked the question. People support abortion when it is rape or incest and if asked the question it would be much higher support. When asked if you support an abortion where the child is born then left alone to die, (Obama definition) then the answer would support pro life.

As an example I would support it for case one and not for case two. So then you get into the argument of when does a fetus become a life. I can't answer that question so it's a difficult position. However it is an issue where I could design a poll question to support either position.