I think it's more she thinks that she supports abortion because she considers herself a liberal and that is sort of the defining chapter in the book of lib. I don't try to convert her , at least not obviously. I make occasional arguments in less important areas that she agrees with and then might mention the stance of one of the parties or a member of one. Usually she is surprised by who she is in agreement with. Eventually I'll show her the Silent Scream video and I think she'll have a change of heart. Hopefully so because I could never be very serious with someone who would support something as horrible as abortion.
The thing is, you don't have to engage her on the subject of abortion at all, AFA the relative pros and cons - the thing to do is (IMO) engage her in *why she thinks THIS ONE ISSUE is so important.
That way you can dig into her (obviously very shallow) psychology and inflict some real damage, where as if you just try to argue the morality of abortion, she'll just find a way to pigeonhole you and your arguments.
The point is, liberal airhead women can argue in favor of abortion, and always find some grounds (in their minds) upon which to base their arguments - but when you start digging into the *relative importance* of it, compared to other issues, their obsession with the issue can't hold up under intellectual scrutiny by any standards - and none of the feminist platitudes they've memorized will help them survive your attack.
I.e., while one can make valid arguments that that First Amendment, or Second Amendment, issues should be used as a litmus test for a political party or candidate, it is impossible, as far as I can tell, to make such an argument for abortion. Nevertheless, that is what these moronic women do - and without knowing they're doing it, much less undersatanding why they're doing it - and consequently they're completely vulnerable intellectually when you attack them from that angle.