NARAL, Planned Parenthood

Why do I always hear about how NARAL and Planned Parenthood have, on a scale of 0-100, rated John McCain a 0 and Barack Obama a 100? Is there any science used in these ratings? Seems pretty binary... if you're a Dem, we'll do anything to get you elected, and if you're in the GOP, we'll do the opposite. Can't we just assume that, in every Dem-Rep election, you'll push onesidedly for the blue team? Given that prolifers think that John McCain, all things considered, is a pretty weak candidate for their cause, what do abortion rights groups see differently?